如图,光滑的四分之一圆弧轨道AB固定在竖直平面内,A端与水平面相切。穿在轨道上的小球在拉力F作用下,缓慢地由A向B运动,F始终沿轨道的切线方向,轨道对球的弹力为N.在运动过程中
Did you______ your mom_______ the garden. A.help; clean B. helped; cleaned C. help; cleaned
I have a headache. My right hand _______. A.hurts B. hurted C. hurts
Mike is_______ me. A.taller B. tall C. taller than
_______ does she feel? A.What B. How C. Where
______ you________ last summer holiday? A.Did; saw elephants B. Do; see elephants C. Did; see elephants