一 Ben and Sue aren't at home, are they?
— No. They to London on business.
A:have gone
B:go
C:have been
D:will go
Look! I have a green bag.
如图,小华用不计重力的弹簧测力计拉着一物块以2 m/s的速度在水平地面上匀速前进,弹簧测力计的读数为____N,物块受到的滑动摩擦力的大小为 ____N;当速度增加到4 m/s时,物块受到的滑动摩擦力的大小为 ____N