Why are scientists unsure which types of gases come from Phobos?
A.No one has been able to see the gas.
B.Their machine broke too soon.
C.The planet"s atmosphere is very thick.
D.There was not any interest in Mars.
第8题
A.轴承内外圈滚道及滚子表面剥离
B. 内圈破裂
C. 套圈和滚子工作表面擦伤
D. 套圈和滚子滚动表面划痕
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