He was just   __ call you up    __you came in,    __  was unexpected. [ ]

A. about to; when; that        

B. to; as; as        

C. to; as; that               

D. about to ;when; which  

在建筑装修中，工人用质量为M的磨石对斜壁进行打磨，当对磨石加竖直向上推力F时，磨石恰好沿斜壁向上匀速运动，已知磨石与斜壁之间的动摩擦因素为μ，则磨石受到的摩擦力是（　　）

A．（F-mg）cosθ

B．（mg-F）sinθ

C．μ（mg-F）sinθ

D．μ（F-mg）cosθ