PrepTest 141

Incorrect. The stimulus establishes that these minerals are how fluoride enters the groundwater, so this answer is contradicted.

Incorrect. The relationship in the stimulus held rainfall constant, thus removing it from the equation. Different levels of rainfall could result in different fluoride levels, but the amount of rain has the same effect on those levels.

Incorrect. There's no comparison between fluoride levels and sodium levels, just a note that high levels of each go hand in hand.

Correct. Argument or Facts:
Facts

Question Type:
Must Be True

Stimulus Summary:
Fluoride in groundwater is from when rain dissolves minerals in soil. Holding all relevant variables constant, higher levels of fluoride in groundwater also have a lot of sodium.

Answer Anticipation:
That second statement creates a relationship that's very likely to play into the correct answer. In stating that "relevant variables" are held constant, the study has shown that sodium is in some way related to the fluoride in groundwater. Since that fluoride enters the groundwater through the dissolution of minerals in the soil (check the phrasing—it's not one source, but how it enters the water), the correct answer will likely connect these two ideas to state that sodium in some way increases the dissolution of fluoride.

Answer Explanation:
This answer connects the high sodium levels where there's high fluoride levels when all else is held constant to the means by which fluoride gets into the groundwater. If the only relevant difference between areas with different fluoride levels is sodium, and the source of fluoride in groundwater is dissolved minerals, then the sodium must act to get that fluoride from the minerals faster.

Key Takeaway:
When an argument sets all other variables as constant, then whatever the difference is must have some influence on any change.

Incorrect. High fluoride is correlated with high sodium levels, not the other way around. This answer reverses the relationship.