We didn't enjoy the picnic ________ there was a sudden rain. [ ]

A. if

B. but

C. because

D. however

如图所示，用恒力F拉着质量为m的物体沿水平面从A移到B的过程中，下列说法正确的是（　　）

A．有摩擦力时比无摩擦力时F做的功多

B．有摩擦力时与无摩擦力时F做的功少

C．物体加速运动时F做的功比减速运动时F做的功多

D．物体无论是加速、减速还是匀速，力F做的功一样多