A dark cloud is a of rain.A. signB. signingC. markD. signature

问题补充：

A dark cloud is a of rain.

A. sign

B. signing

C. mark

D. signature

答案：

(答案→)A. sign

B. signing

C. mark

D. signature

解析：考查词义辨析。sign 符号，迹象，mark标记，signature 签名，句意：乌云是一种下雨的迹象。所以选A。