PrepTest 156

This doesn't match our anticipation that the correct answer will tell us that we can calculate planets' orbits by calculating their gravitational force. In fact, this answer choice doesn't tell us anything about calculating planetary orbits, allowing us to eliminate this quickly.

Besides, this answer choice doesn't make the conclusion valid. This answer choice only tells us there's a mutual dependence between mass and gravitational force. Gravitational force depends on mass (according to the passage), and mass depends on gravitational force (according to (A)). But that doesn't tell us anything about our ability to calculate planetary orbits, so this doesn't make the conclusion valid.

Here it is, an answer choice that connects calculating gravitational force and calculating planetary orbits. In fact, this answer choice is a perfect match for our anticipation. If calculating planetary orbits "requires considering only" their gravitational forces, then all we have to do to calculate two planets' orbits is calculate their gravitational forces. In other words, if we calculate two planets' gravitational forces, then we'll calculate their orbits. And since calculating their gravitational forces only requires knowing the planets' masses and the distance between them, we don't need "a theory of the structure and constitution of the Sun and the planets in order to calculate their orbits."

Since this makes the conclusion valid and matches our anticipation perfectly, we can select (B) and wrap up this question.

This doesn't match our anticipation that the correct answer will say that calculating planets' gravitational force will allow us to calculate their orbits. Indeed, this answer choice doesn't tell us anything about calculating planetary orbits, so we can immediately eliminate it.

Besides, this answer choice doesn't make the conclusion valid. The conclusion is only about what is true "according to the Newtonian theory." Even if the Newtonian theory is wrong about everything, that wouldn't affect the conclusion's validity. In other words, this argument only cares about what's true according to the Newtonian theory, not what's actually true.

This doesn't match our anticipation that the correct answer will say that if we calculate planets' gravitational force, then we can calculate their orbits. In fact, this answer choice doesn't tell us anything about calculating planetary orbits, so we can cross it off without giving it further thought.

Besides, this answer choice doesn't make the conclusion valid. This answer choice says that if we know what an object is made of, then we can determine its mass. But that won't help us validate a conclusion that asserts that we don't need to know what planets are made of to calculate their orbits.

This doesn't match our anticipation that the correct answer will tell us that calculating the gravitational force between two planets will enable us to calculate their orbits. In fact, this answer choice doesn't tell us anything about calculating planetary orbits, allowing us to eliminate this quickly.

Besides, this answer choice doesn't make the conclusion valid. This answer choice tells us there's a mutual dependence between distance and gravitational force. Distance is a factor in determining gravitational force depends on distance (according to the passage), and gravitational force is a factor in determining distance (according to (E)). But that doesn't tell us anything about our ability to calculate planetary orbits, so this doesn't make the conclusion valid.