Scientists have found no of life on Mars so far though great efforts have made on the rese

问题补充：

Scientists have found no of life on Mars so far,though great efforts have made on the research.

A. symbols

B. signs

C. marks

D. signals

答案：

A. symbols

(答案→)B. signs

C. marks

D. signals

解析：signs 迹象；symbols 象征；marks 标志；signals 信号。