He is not easy toget along with, but the friendship of his, ________, will last forever.
A.oncegainedB.to begainedC.aftergainedD.whilegaining在线课程A
解析：
gain与friendship是被动关系，用过去分词。译文：他很难相处，但是一旦获得了他的友谊，就永远不会失去了。