Which Issue Presents an Obstacle to Controlling Drugs Via Source

Which issue presents an obstacle to controlling drugs via source control strategies?

A) The drug trade is an important source of foreign revenue and source control undermines third-world economies.
B) The amount of foreign grown narcotics is so vast that even if 75 percent of the opium market were destroyed, the United States would still require only 10 percent of the remainder to sustain its drug trade.
C) Even if the government of one nation were willing to cooperate in vigorous drug suppression efforts, other nations would be eager to cash in on the drug trade void.
D) These are all issues that present obstacles to controlling drugs via source control strategies.