单项选择题

A．They never occur.
B．They are net affected by the force of gravity.
C．In still air they would fall to earth.
D．In moving air they fall at a speed of thirty-two miles per hour.
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单项选择题
A．They are kept aloft by air currents.B．They combine with other chemicals in the atmosphere.C．Most of them evaporate.D．Their electrical charges draw them away from the eart
A．They are kept aloft by air currents.
B．They combine with other chemicals in the atmosphere.
C．Most of them evaporate.
D．Their electrical charges draw them away from the eart
单项选择题
A．They are good friends.B．They are former schoolmates.C．They are neighbors.D．They are former workmates.
A．They are good friends.
B．They are former schoolmates.
C．They are neighbors.
D．They are former workmates.
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