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

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

Explanation:

Mexico and the United States share a 3154 km long border, this along with the fact that form that border is from where the southamericans and central americans immigrate illegaly to the states and the legal and illegal transport of goods on the border have strained the border realtions between the two countries.

the answer would be choice c, cave paintings.

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