31. I ______ to reach the manager for the last two hours but the line is always busy.
A
tried
B
have tried
C
had tried
D
have been trying
现在完成时的结构是 have/has + been + doing ；故选项D正确。
从后面的“but the line is busy”可见，“我”试着打电话应该是从两小时之前一直持续到了现在。
对现在完成时的定义及用法不熟悉致错。