The presence of blue-green algae blooms i

Hello, warrior1991. (A) is the only defendable answer, since it is the one that correctly incorporates what is known as an absolute phrase, the ambiguous their notwithstanding. These questions give GMAT™ aspirants plenty of grief, from what I have observed on the forum. My guess is that the problem lies in the phrase itself appearing to be an aborted clause. However, an absolute phrase modifies the whole sentence and should be missing a main verb in the following structure:

[noun] + [participle] + [optional modifiers or objects]

The presence of blue-green algae blooms in Lake Erie has disrupted regular water supply for over 400,000 people in Ohio, their numbers increasing to hazardous levels as a result of polluted runoff from industries and farmlands and warm temperatures.

Answer choices (B), (C), and (D) all place a verb after numbers, creating what is known as a comma splice—a grammatically incorrect sentence in which a comma stands in for a period or semicolon. That is, you cannot join two independent clauses with nothing more than a comma (i.e. without a conjunction). Thus, the only grammatical sentences are (A) and (E). Between the two, the latter part of (E), the and, should make it an easy elimination.

I do not think the sentence would appear as is on the GMAT™, but you should know how to identify an absolute phrase, since, again, they pop up now and again in 700-level questions.

I hope that helps. Thank you for thinking to ask me.

- Andrew