进口货海关放行的单证是（）。

A.提货单

B.提单

C.关单

D.大副收据

--- I have great difficulty figuring out the answer to this problem.   

--- _____ my advice, you ____ less difficulty with such problems. 

A．If you had followed, would have had

B．If you followed, would have

C．If you had followed, would have

D．If you listen, will have