THE CORRECT ANSWER IS (B). THERE ARE TWO PROBLEMS WITH THE ORIGINA...

21. The correct answer is (B). There are two problems with the original version. First, is

proved is an improper verb form (is proven is the correct present-perfect form). Also, be

a cause of is wordy. Choice (B) corrects both problems. So does choice (D); however, the

resulting sentence is nonsensical, suggesting that drugs (rather than researchers) do

the proving.