单项选择题

What is the characteristic of the Martian surface according to observations over the past 20 years

A．Remaining stable all the time.
B．Appearing bright red.
C．Looking dark most of the time.
D．Changing from bright to dark or vice vers
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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
单项选择题
What is the summertime polar cap composed of
A．Dust.
B．Sand.
C．Water ice.
D．Volcanic rocks.
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