单项选择题

How is the image of the north polar cap of Mars

A．Vague.
B．Clear.
C．Cannot be seen.
D．Barely visibl
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单项选择题
What is the summertime polar cap composed of
A．Dust.
B．Sand.
C．Water ice.
D．Volcanic rocks.
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.
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