Continental tropical air masses centered over northern mexico have little influence on weather in the united states because they
a. never influence weather
b. rarely move outside their source region
c. only move toward the equator
d. are too high in altitude

b. rarely move outside their source region

Explanation:

As continental tropical air masses formed in the interior of the subcontinents they tend to be stagnant. As they are hot and dry they tend to form over lands.  The northward movement of the air mass teds to bring moisture in north America and hence produce rainfall. But they rarely extend over the north or east of southern California and originate over the Gulf of Mexico and the Caribbean sea.

Earths place in space