小小解说员（解释词语）。

1．束手无策：                                                                                                                                 

2．垂头丧气：                                                                                                                                 

3．胸有成竹：                                                                                                                                 

地高辛的作用机理是

A．70%～75%
B．30%～35%
C．60%
D．45%
E．30%