Video Explanation

Text Explanation

Choice (A) follows the exact pattern of the idiom discussed in this blog --- first verb ("Had … allowed") in the subjunctive and second ("annexing") as a gerund, and the independent clause after the comma is in the subjunctive. This is quite promising.

Choice (B) uses the preposition "with" as a quasi-subordinate conjunction, with the construction "with" [noun][participial phrase]. The GMAT considers this incorrect 100% of the time. Also, the independent clause after the comma is in the indicative mood, giving the mistaken impression that the "California nation" actually existed. (B) is not correct.

Choice (C), up to the comma, is promising --- a factual statement of what the US did and didn't do, and the infinitive of purpose in the second half is fine, but unfortunately, this choices makes an idiom mistake. The idiom with "to prevent" is "to prevent A from doing X", NOT "to prevent A to do X." Because of this idiom mistake, (C) is not correct.

Choice (D) is an example of false parallelism, putting the three verbs of the sentence into the grammatical relationship of parallelism without considering their logical connection. Everything is in the indicative, so the hypothetical nature of the "California nation" is lost. Perhaps most significantly, this makes a major pronoun mistake: "it annexed it" --- a repeated pronoun for two different antecedents. (D) is not correct.

Choice (E) has makes double subject mistake --- "The US [long modifier] it prevented …" Either "the US" or "it" has to be the subject of "prevented" --- both cannot be the subject simultaneously. Also, " … annexing it, it prevented …" --- this choice makes the same kind of repeated pronoun mistake found in (D). (D) is not correct.

The only possible answer is (A).