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
网友回答
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。