He is a key in the murder so we must protect him from all kinds of danger.A. witchB. watc

问题补充：

He is a key in the murder so we must protect him from all kinds of danger.

A. witch

B. watcher

C. wizard

D. witness

答案：

A. witch

B. watcher

C. wizard

(答案→)D. witness

解析：词义辨析。A.巫婆 B.观察者 C.术士 D.证人，目击者。句意：他是这起凶杀案中的关键证人，因此必须保护他避免各种危险。故选D。