He is rather difficult to make friends with, but the friendship of his, ________ is more true than any other.[ ]

A．once gained  

B．when to gain

C．as gained  

D．while gaining

在T细胞表面与TCR结合，传导T细胞活化信号的分子是（）

A.CD79a

B.CD3

C.CD28

D.CD8

E.CD4