He was about to say something more _____ a boy rushed in, gun in hand. [ ]

A. that

B. when

C. in case

D. while

将重30N的物体放在水平地面上。现用10N水平向右的力拉着该物体以1m/s的速度作匀速直线运动，则物体所受的摩擦力为______N，方向是_________。