A higher interest rate is only one of the factors, albeit an important one, that keeps the housing market from spiraling out of control, like it did earlier in the decade.
a. keeps the housing market from spiraling out of control, like it did
b. keep the housing market from spiraling out of control, as it did
c. keeps the housing market from spiraling out of control, as it did
d. keep the housing market from spiraling out of control, like
e. keep the housing market from spiraling out of control, like it did

Here - the ans given is B - and the explanation is as follows:
The original sentence states that "a higher interest rate is only one of the factors...that keeps..." The clause "that keeps the housing market from spiraling out of control" is describing the word "factors." Since "factors" is plural noun and "keeps" agrees with singular nouns, we need to find a sentence that replaces "keeps" with "keep". Also, "like it did earlier in the decade" is incorrect. When comparing clauses (i.e., a phrase containing a subject and a verb), we must use "as" instead of "like." For example, "She sings like her mother" and "She sings as her mother does" are both correct, but "She sings like her mother does" is not.

Is'nt "one of the factors" a prepositional phrase and should'nt the singular subject match the singular verb "keeps"? Thought the answer should be C...

Check this out: http://www.manhattangmat.com/forums/post1047.html
and let us know if that doesn't help