一道GMAT的CR题,The program to control the entry of illegal drugs

一道GMAT的CR题,
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.
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 substantial 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.
the answer is (E)