Logical reasoning PrepTest 156 · Section 4 · Question 20

This strengthens the argument, but it isn't necessary.

This answer choice strengthens the argument by reinforcing the premise that clay soil can be mixed with other soils and create an acidic soil. This answer choice confirms that this will work if we mix clay soil with any acidic soil.

However, this isn't necessary to the argument because we don't need to know that adding acidic soil to any type of soil will create acidic soil. The conclusion only says that azaleas can thrive in clay soil. So, we just need to know that adding at least one soil to clay soil will make the clay soil acidic. We don't need to know anything about other types of soil that could be mixed with acidic soil.

Alternatively, this is the argument we'd make if we used the backup plan (note that we simplified the premises and conclusion and negated this answer choice):

"Azaleas flourish in acidic soil. Clay soil is rarely acidic. One can create acidic soil by mixing clay soil with other types of soil. But, mixing acidic soil with another type of soil won't necessarily result in acidic soil. So, azaleas won't flourish in clay soil."

This doesn't sound like a compelling argument. The part that follows "But" doesn't contradict the previous statements. Even if mixing acidic soil with another type of soil won't necessarily result in acidic soil, we know that mixing clay soil with another soil will create acidic soil. So, azaleas could still flourish in clay soil, contrary to what this conclusion states.

No matter which approach we use, we can confidently cross off (A).

As we anticipated, this answer choice strengthens the argument by establishing that clay soil mixed with another type of soil can still be considered clay soil. That supports the conclusion that azaleas can flourish in clay soil. For azaleas to thrive in clay soil, we need to mix clay soil with other types of soil to make the clay soil acidic. This answer choice tells us that this acidic clay soil mix is still clay soil, confirming that azaleas can flourish in clay soil.

We also need to know that this acidic clay soil mix is still clay soil to draw the conclusion. If this acidic clay soil mix could not be considered clay soil, then we wouldn't have any evidence that azaleas could thrive in clay soil. After all, regular clay soil isn't acidic, so azaleas wouldn't flourish in that soil. Since this answer choice strengthens and is needed to draw the conclusion, it is correct.

Alternatively, this is the argument we'd make if we used the backup plan (note that we simplified the premises and conclusion and negated this answer choice):

"Azaleas flourish in acidic soil. Clay soil is rarely acidic. One can create acidic soil by mixing clay soil with other types of soil. But, clay soil mixed with other types of soil is not clay soil. So, azaleas won't flourish in clay soil."

This makes sense. If clay soil mixed with other types of soil cannot be considered "clay soil," then azaleas won't flourish in clay soil. Regular clay soil is "rarely acidic," but azaleas flourish in acidic soil. Therefore, azaleas won't do well in non-acidic clay soil. Since this argument makes sense, (B) also passes the backup test.

So, no matter which approach we used, we can be confident that (B) is the correct answer. We'd be justified in selecting (B) and advancing to the following question.

This strengthens the argument, but it isn't necessary.

Like (A), this answer choice strengthens the argument because it verifies the premise that clay soil can be mixed with other soils to create acidic soil. This answer choice shows that we can make acidic soil by mixing clay soil with any soil.

However, this isn't necessary to the argument because we don't need to know that adding clay soil to any type of soil will create acidic soil. The conclusion only says that azaleas can thrive in clay soil. To prove that conclusion, we just need to know that there's one type of soil that we can add to clay soil and make acidic clay soil. This answer choice, therefore, provides far more information than we need.

Alternatively, this is the argument we'd make if we used the backup plan (note that we simplified the premises and conclusion and negated this answer choice):

"Azaleas flourish in acidic soil. Clay soil is rarely acidic. One can create acidic soil by mixing clay soil with other types of soil. But, mixing clay soil with some other types of soil won't produce acidic soil. So, azaleas won't flourish in clay soil."

This conclusion isn't persuasive. Even if mixing clay soil with some soils won't produce acidic soil, the earlier premise says that mixing clay soil with other soils will. So, it's still possible that azaleas will flourish in clay soil, contrary to what this conclusion states.

No matter which approach we use, we can confidently cross off (C).

This doesn't strengthen the argument. The conclusion is only about azalea bushes, so we only care about those plants. We don't care whether other plants don't flourish in acidic soil.

Alternatively, this is the argument we'd make if we used the backup plan (note that we simplified the premises and conclusion and negated this answer choice):

"Azaleas flourish in acidic soil. Clay soil is rarely acidic. One can create acidic soil by mixing clay soil with other types of soil. But, plants flourish in acidic soil. So, azaleas won't flourish in clay soil."

This argument is nonsensical. The part that follows "But" doesn't contradict or challenge what came before it. So, inserting the part that follows "But" makes a jarring and confusing argument. Moreover, the argument doesn't explain why azaleas won't flourish in clay soil.

No matter which approach we use, we can confidently cross off (D).

This strengthens the argument, making this a very tempting answer choice. In fact, this answer choice provides the information we're looking for. It confirms that "treated" clay soils (presumably, ones mixed with other soils to become acidic) are still clay soils. And it shows that azaleas can flourish in clay soils. That confirms that this conclusion is true!

So, why is this answer choice wrong? Because we don't need to know this information to draw the conclusion. The conclusion didn't argue that azalea bushes can flourish just as much in clay soil as they would in other acidic soils. The conclusion only said azalea bushes can flourish in clay soil. Even if azalea bushes do a little bit worse in clay soils, they can still "flourish" in those soils.

Alternatively, this is the argument we'd make if we used the backup plan (note that we simplified the premises and conclusion and negated this answer choice):

"Azaleas flourish in acidic soil. Clay soil is rarely acidic. One can create acidic soil by mixing clay soil with other types of soil. But, Azaleas don't do as well in treated clay soils as in naturally acidic soils. So, azaleas won't flourish in clay soil."

This argument isn't terrible, but it's not as persuasive as (B)'s. The fact that azaleas don't do as well in clay soils doesn't imply that azaleas do poorly in clay soils. So, even if Azaleas don't do as well in treated soils as naturally acidic soils, they can still flourish in clay soil, contrary to what this conclusion states.

No matter which approach we use, we can confidently cross off (E).