_____ many times, but he still couldn't understand it. [ ]

A. Though he had been told

B. Having been told

C. Having told

D. He had been told

组成蛋白质二级结构的化学键是（）

A.氨基酸的α-羧基与相邻氨基酸α-氨基脱水缩合成肽键

B.肽键与肽键之间形成氢键

C.侧链间的氢键

D.次级键

E.游离氨基与羧基形成肽键