Mexico and the united states have strained border relations mainly due to the following factor:
a. mexico is the largest foreign supplier of marijuana and methamphetamine to the us market
b. abundant rainfall in recent years along much of the mexico-us border region has ameliorated periodically strained water-sharing relationships
c. mexico must deal with thousands of impoverished guatemalans and other central americans who cross the border looking for work
d. the us has intensified security measures to monitor and control legal and illegal transport of people and goods