问题补充：
Many scientists have believed that there may be a of life on the Mar．
A. mark
B. signal
C. sigh
D. sign
答案：
A. mark
B. signal
C. sigh
(答案→)D. sign
解析：考查名词辨析：A. mark记号，B. signal信号，C. sigh叹息，D. sign标志，迹象，句意是：很多科学家一致相信在火星上可能有生命的迹象。选D。
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