单选题
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.
【正确答案】
D
【答案解析】
[注释] 见第一段第五句The images were taken in the middle of the Martian northernsummer，when the polarcap was at its smallest size．
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