He had to___ before they finished the course because they were not well disciplined.

A．drop in

B．drop out

C．drop off

D．drop by

既不取得商品所有权，也不实际取得商品，而只在买卖双方之间从事介绍、撮合，促进双方成交的商人称作（）。

A.销售代理商

B.采购代理商

C.经纪人

D.拍卖商