题目

--Mom，could I have an MP3 like this? --Certainly，we can buy ________ one，but it's as good as this．The price of this kind is a little high． A．a cheap          B．a cheaper                  C．a small              D．a smaller 答案：B用力F=40N，按住一重100N的木块，如图所示，当木块沿竖直方向匀速下滑时，木块受到的摩擦力的大小是 N．