单选题
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
【正确答案】
D
【答案解析】
[注释] 见第二段第三句Some regions that were dark 20 years ago are now bright red； some areas that were bright red are now dark．
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