Video Explanation

Text Explanation

Split #1: Idioms

This sentence features two verbs with similar meanings but two completely different idiom patterns.  The verb “to prohibit” always takes “from” + [gerund]; using the infinitive is 100% incorrect with this verb.  By contrast, the verb “to forbid” always takes the infinitive; using “from” + [gerund] is 100% incorrect with this verb.

Choice (A): “prohibit … from … forbid … to” = correct

Choice (B): “prohibit … to … forbid … to” = idiom mistake

Choice (C): “prohibit … from … forbid … from” = idiom mistake

Choice (D): “prohibit … from … forbid … from” = idiom mistake

Choice (E): “prohibit … to … forbid … from” = double-whammy idiom mistake

Even though this split isolates the correct answer, I will explore other grammatical issues in this question.  

Split #2: the Inside/Outside rule

One important rule about parallelism is the Once Outside, Twice Inside rule.  This rule comes into effect when we have correlated conjunctions—that is, a coupled pair of conjunctions marking the two halves of the parallelism: examples include “both X and Y,” “neither X nor Y,” “not X but Y,” and “not only X but also Y.”  When any of these are used, the parallelism has a clear start at the beginning of the first word. Anything that is within the “both X and Y” structure is “inside,” and anything that comes before or after it is “outside.” Now, suppose a preposition applies to words in both branches of the parallelism.   We have two options:

Once Outside: to both X and Y

Twice Inside: both to X and to Y

In this question, the parallel elements are the infinitive “to prohibit … and to forbid.”  Thus, we could have:

Once Outside: to both prohibit and forbid

Twice Inside: both to prohibit and to forbid

The incorrect patterns are

Once inside: both to prohibit and forbid*

Once outside once inside: to both prohibit and to forbid

Choice (A): twice inside = correct

Choice (B): twice inside = correct

Choice (C): once outside, once inside = incorrect

Choice (D):  twice inside = correct

Choice (E): once outside, once inside = incorrect

Split #3: parallelism

The “both X and Y” structure is a standard parallelism frame.  In this structure, X and Y need to be in parallel: two infinitives.  

Choice (B) violates the parallelism: “both to prohibit … while forbidding.”  Choice (B) is incorrect.

For a variety of reasons, the only possible answer is (A).  

*NOTE: with the special case of infinitives in parallel, super-purists would say that the otherwise correct “once outside” pattern creates a split infinitive.  A “split infinitive” occurs when any word comes between the “to” and the verb of an infinitive (e.g. to boldly go where no man has gone before).  Grammatical conservatives (including Mike McGarry) object to split infinitives in all cases.  Folks who are grammatically more liberal (such as Chris Lele) think they’re perfectly permissible under certain conditions.  The GMAT takes no position on these: in general, the GMAT likes to avoid controversy, so typically split infinitives only appear in SC answer choices that are wrong for other clear reasons.   As split infinitives are gaining wide support in many well-educated circles, it’s best not to draw any conclusion about them on the GMAT. In your own writing, you get to be as sophisticated or casual as you like.