He is not easy-going but his trust in his friends will last forever once .A. to be gained

问题补充：

He is not easy-going, but his trust in his friends will last forever once .

A. to be gained

B. is gained

C. being gained

D. it is gained

答案：

A. to be gained

B. is gained

C. being gained

(答案→)D. it is gained

解析：句意：他不是随和的人，但是他对朋友的信任，一旦被获得就永远不会失去的。Once引导的是条件状语从句，trust和gain是被动关系，而且条件状语从句用一般现在时代替一般将来时，所以选D。