Identifying Sentence Errors
Correct Answer: C
The error in this sentence is (C): the verb "has" (which is singular) does not agree in number with the noun "effects" (which is plural). The noun phrase "harmful effects" is the subject of the verb phrase "has become known." The correct sentence reads: Air pollution caused by industrial fumes has been studied for years, but only recently have the harmful effects of noise pollution become known.
(from the October 12, 1999 test)