Such a problem still exists because no adequate measures for preventing it _____ taken in the past.

A. are
B. had
C. were
D. had been.
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科目：高中英語 來源：專項題 題型：單選題

Such a problem still exists because no adequate measures for preventing it _____ taken in the past.
[     ]
A. are
B. had
C. were
D. had been.

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