已知最初水平、最末水平及时期数，要计算平均发展速度，应采用（）。

A.水平法

B.累计法

C.两种方法都能采用

D.两种方法都不能采用

Reviewing the video monitor afterwards, they noticed two armed men with their faces covered by masks

step out of a carriage and ______the bank through the backdoor.     [ ]

A. slide into    

B. enter into    

C. stole into    

D. went into