Susan ______ me a letter since last summer. It  will be a surprise to receive her letter.   [ ]

A. hasn't sent  

B. hadn't sent

C. has sent    

D. had sent

发动机油中添加的清净分散剂主要用于吸附机油中的（）。

A．低温油泥

B．固体污染颗粒

C．漆膜