When I was in high schoool, the teachers were always warning

When I was in high schoool, the teachers were always warning us against ever using drugs. Yet many of the students freely admitted in later years that they had used drugs regularly in high school. I can only draw the unfortunate conclusion from this that my teachers' warnings were in vain.

Which one of the following claims, if true, would be most useful in refuting the argument of the passage above?

A) Many of the students who used drugs were aware of the dangers involved because of the teachers' warnings.

B) Some students use drugs for legitimate reasons.

C) The school had to compel teachers to warn their students against drug use.

D) Some of the author's fellow students were persuaded not to use drugs by their teachers' warnings.

E) Drug use invariably results in bad study habits and poor class attendance.