Explanation

Creating a filter: the original sentence has an error at the beginning of the underlined portion. As written, the clause "which is dethroned" modifies "customers," contrary to the intended meaning of the sentence. To keep it in reference to the about-to-be-dethroned social network, we must cut the comma from the phrase and add a conjunction, such as "and." We'll look for an answer choice that resolves this defect.

Applying the filter: answer choices (C) and (E) both roughly match what we are looking for. Choice (D) has the same problem as (A), and choice (B) creates a clause without a grammatical subject.

Finding objective defects: we are left with choices (C) and (E). Choice (E) draws us away from the intended meaning. "Dethrone" indicates that someone else is taking the throne, but (E) does not convey clearly that the new company is taking the throne. Also, the word "then" in (E) is inferior to the conjunction "but" in (C), because it fails to express the contrast between the two events in the pattern. The correct answer is (C).