Of the following, which is the most significant factor in causing strained border relations between Mexico and the United States?

Mexico is the largest foreign supplier of marijuana and methamphetamine to the US market

Abundant rainfall in recent years along much of the Mexico-US border region has ameliorated periodically strained water-sharing relationships

Mexico must deal with thousands of impoverished Guatemalans and other Central Americans who cross the border looking for work

The US has intensified security measures to monitor and control legal and illegal transport of people and goods