A．Because the U.S. government encourages them to do so.
B．Because they are driven by others.
C．Coca is popular in Colombia.
D．Because they can make more money from growing coca.

A．It's defined as the result of the moisture in the Earth's atmosphere.
B．It's defined as the result of the Earth's rotation.
C．It's defined as the horizontal movement of air.
D．It's defined as the vertical movement of air.

A．It's the ultimate cause of winds.
B．It causes vertical movements of air.
C．It reduces differences in air pressure.
D．It's used to predict weather patterns.