Drug ___1___ officers in the United States said they have arrested more than 2,000 people in a lengthy investigation targeting Mexican trafficking rings. They said the ____2___also involving the FBI, the Mexican government and other American agencies has last almost two years and commentated on Wednesday with more than 400 arrests across 16 states. The Assistant Secretary and Director of Customs Enforcement at the Department of Homeland Security John Morton said the United States was serious about ____3___of drug crime.

　　"We face ___4____ along our border with Mexico. The drug cartels there are brethren, the ailing smugglers, the money launders, the gun traffickers all make a living ____5___US and Mexican law on a daily basis. It's a violent business, it's a corrupt business. It's a business that demands our full attention. As you see here, it has our full attention."

　　答案：

　　enforcement

　　joint operation

　　tackling the problem

　　a serious, sustain(ed) challenge

　　violating