单选题He is rather difficult&nb

问题补充：

单选题Heisratherdifficulttomakefriendswith,buthisfriendship,____,ismoretruethananyother.A.once gainedB.when to gainC.after is gainedD.while gaining

答案：

A解析once gained是状语从句的省略, 相当于once it is gained。