My teacher takes great trouble ______me learn English, since I have a lot of trouble _____it.      [ ]

A. helping; learning  

B. to help; to learn  

C. helping; to learn  

D. to help; learning

有一工厂车间，吸声降噪处理前某频带的混响时间为4s，吸声降噪处理后该频带的混响时间变为1s，则该频带的室内噪声声压级可降低（）dB。

A.3

B.6

C.8

D.10