I ____ to reach the manager for the last two hours but the line is always busy. [ ]

A. tired

B. have tired

C. had tired

D. have been trying

化学发光剂的标记方法不包括（）.

A.碳二亚胺缩合法

B.过碘酸钠氧化法

C.重氮盐偶联法

D.N-羟基琥珀酰亚胺活化法

E.基因工程法