UPDATE: You can find this blog and others about idioms in our new GMAT Idiom eBook!
First of all, try this Sentence Correction question. A full explanation will follow later in the post.
1) Whereas both Europe and China use standard railroad gauge (1435 mm), Russia deliberately chose the wider “Russian gauge” (1520 mm) that gives greater side-to-side stability in railways cars and, more importantly, acts as a national defense, so that it would block foreign army’s supply line and preventing these bordering powers from invading by train.
(A) acts as a national defense, so that it would block
(B) acts like a national defense, so as to block
(C) acts as a national defense, blocking
(D) acting as a national defense, blocking
(E) acting like a national defense, would block
As I explain in the post linked above, in general “like” is followed only by a single noun, and is used to compare nouns; but “as” is followed by a full noun + verb clause, and is used to compare actions.
2) This rookie swings like Ted Williams.
3) Ted Williams leads the majors in career on-base percentage, as Babe Ruth leads in career slugging percentage. Each is in second place behind the other on the respective lists.
The Idiom: “act like”
You will get a lot of mileage out of the general rule for “like” vs. “as”, but it is no longer a reliable guide when you get to this idiom.
In English, the idiom “to act like” means to behavior or comport one’s self in imitation of something else. If I “act like a king”, that implies that I am not a king, but something about my behavior (presumably, my entitlement and presumption) resembles that of a king. A person is capable of intending to imitate something, so a person can “act like” something. Conceivably, an intelligent animal (one of the higher primates, for example) could be induced to imitate something, in which case we could say: the chimpanzee “acts like” such-and-such. Any inanimate object is utterly devoid of intentionality, so we cannot in any way attribute imitative behavior to it: therefore, we can never use the idiom “act like” with an inanimate object. With an object, we always have to use “act as.”
Explanation of the Question
First of all, from the foregoing discussion, we know that the inanimate object “Russian gauge” cannot “act like” anything, because it doesn’t have the conscious ability to imitate. If the subject is an inanimate object, we need to use “act as”. Thus, (B) and (E) are out immediately.
We also have two parallel constructions we need to maintain here. We need the two verbs following “Russian gauge” to be in parallel —- the first is “gives”, so the second has to be the parallel “acts” —- thus, (D) is out.
The second parallelism is between “would block”/”blocking” and the participle “preventing”; clearly, we need the participle “blocking” for the first verb. Therefore, (A) is out, and the only correct answer remaining is (C).