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单项选择题

Which of the following is TRUE about the north polar cap of Mars in summertime

A．It is at its largest size.
B．It is changing its shape all the time.
C．It is at its smallest size.
D．It is at its windiest tim
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单项选择题When were the images of Mars taken

A．In the middle of summer on Earth.
B．In the middle of summer on Mars.
C．In the middle of summer on Earth's polar caps.
D．In the middle of summer in the northern part of Mars.

单项选择题What is the summertime polar cap composed of

A．Dust.
B．Sand.
C．Water ice.
D．Volcanic rocks.