In the early 1800s, seven planets were known, but perturbations in the

Magoosh Official Explanation:

A question about the events that led to the discovery of Neptune in 1846.

Split #1: the famous missing-verb mistake. In the overall organization of the sentence, we have a first independent clause, “seven planets were known,” then the conjunction “but,” which signals another independent clause coming. For this second independent clause, we get a noun for the subject, “perturbations in the orbit of Uranus,” and we need a full bonafide verb. Choices (A) & (B) give us a participle, and choice (C) gives us an infinitive, but these won’t do. We need the full verb “suggested,” which appears in (D) & (E). We can eliminate (A) & (B) & (C).

Split #2: One kind of logic split involves the choice between an adjective and its corresponding adverb. Here, we have to choose between “hypothetical” and “hypothetically.”

As an adjective, it would have to modify the noun. If we say “a hypothetical eighth planet,” then we are saying that we don’t know whether the planet exists, but if it did exist after the first seven, of course it would be the eighth.

As an adverb, it would have to modify the adjective. If we say “a hypothetically eighth planet,” then we are saying that we are not sure where the planet would fall in the numbering system: it could be the eighth or could be some other number; in this phrasing, it sounds as if the numbering is in doubt, but not the existence of the planet itself.

What was actually in doubt at that moment in history was the planet itself. Of course, if it came after the first seven planets, it would be the eighth: that much was not in doubt. The existence of the planet was in doubt. The adjective “hypothetical” reflects this meaning. We can reject the choices with “hypothetically,” choices (A) and (E).

The only possible answer is (D).