Many scientists have believed that there may be a of life on the MarA. markB. signalC. si

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。