Progesterone, the “hormone of pregnancy,” plays an enormous role in th

Magoosh Official Explanation:

A question about progesterone, an important hormone in the human female reproductive cycle.

Split #1: Idiom

This question tests a subtle idiom difference, “act like” vs. “act as.” Both are correct in different context. The idiom “act like” implies conscious intention, so typically only a human could “act like” something—the implication is that if I “act like” X, I am not really X. For example, I could “act like” an authority on all scientific matters, even though I am not. We use “act like” only for people.

The idiom “act as” is for inanimate objects. It is used when an inanimate object with one usual or typical or regular role takes on some new and perhaps unexpected role:

“Sometimes exercise acts as an antidepressant.”

“At one time, knowledge of baseball acted as a guarantor of the authenticity of one’s claim to American citizenship.”

“Rent control acts as a deterrent to building more housing.”

Here, we are talking about the hormone progesterone, so we need to use “act as.” Choices (B) & (D) make the idiomatic mistake of using “act like”: both of those choices are incorrect.

Split #2: Parallelism, Part One

We have parallel verbs “it . . . crosses . . . and”—we need another full verb, “acts” to complete this parallelism. Choices (D) & (E) use the participle “acting” and fail to complete the parallelism properly. Both of those choices are incorrect.

Split #3: Parallelism, Part Two

We an element of parallelism at the end of the underlining. After the underline section, we have “and diminishing,” so we need a first participle to match this second one. We need “enhancing,” which correctly completes the parallelism. The full verb “enhances” in choice (A) does not complete the parallelism correct. Choice (A) is incorrect.

The only possible choice is (C).