He is rather difficult to make friends with but his friendship is more true than anyone

问题补充：

He is rather difficult to make friends with, but his friendship, is more true than anyone else’s.

A. Once gaining

B. Once gained

C. Once is gaining

D. Once he gained

答案：

A. Once gaining

(答案→)B. Once gained

C. Once is gaining

D. Once he gained

解析：这题考察条件状语从句，当主从句的主语一致时，可以省略从句的主语和be动词，once gained=once his friendship is gained，选B。