OG10-80题的逻辑推导实在是晕啊!请教一下各位牛人!

The program to control the entry of illegal drugs into the country was a failure in 1987. If the program had been successful, the wholesale price of most illegal drugs would not have dropped substantially in 1987.

80.

The argument in the passage depends on which of the following assumptions?

(A) The supply of illegal drugs dropped substantially in 1987.

(B) The price paid for most illegal drugs by the average consumer did not drop substantially in 1987.

(C) Domestic production of illegal drugs increased at a higher rate than did the entry of such drugs into the country.

(D) The wholesale price of a few illegal drugs increased substantially in 1987.

(E) A drop in demand for most illegal drugs in 1987 was not the sole cause of the drop in their wholesale price

在听费费讲义的时候,对这道题有个疑问,想请教一下各位G友,          如果计划不失败,则价格不会下降.    那么根据逆否命题,由价格下降可以推出计划失败.这样能否推出计划失败就是价格下降的原因啊!