单项选择题

Most cloud formations occur when air masses of different temperatures collide.

A．touch
B．smash
C．bump
D．clash
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单项选择题
Pure feldspar is a colorless,transparent mineral,but impurities commonly make it opaque and colorful.
A．truly
B．rapidly
C．periodically
D．frequently
单项选择题
Cattle graze on the dry uplands of the island of Hawaii.
A．wander
B．breed
C．feed
D．exercise
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