Quote:
The town’s legislators heard arguments from the crowd about how town facilities and parks no longer properly accommodate the townspeople now that the number of residents have increased so considerably.
(A) the number of residents have increased so considerably
(B) the numbers of residents have so considerably increased
(C) the number of residents has increased so considerably
(D) the numbers of residents have so considerably increased
(E) the number of residents has been increasing considerably so
The right answer is C. The main concepts tested by this question are Subject Verb Agreement, and Tenses. It gets a whole lot easier once you identify that "the number of x" is always going to be singular since the focus of the subject is not 'x' but the concept of the number itself. This is not dissimilar to "
The number on the scoreboard goes up every time they score" (note the singular 'goes')
Option A, B, D - These three options are all incorrect because they use the plural conjugation "have".
Option E - "has been increasing" is extremely awkward, and uses a perfect-continuous tense not tested on the GMAT. It is simply enough to say that the number "has increased", which is the current use of the present perfect tense (action in the past, reference point in the present).
The right answer is therefore C.