一杯子装满水,杯的底部有一个小孔,在水从小孔不断流出的过程中,杯连同杯中水的共同重心将()
A.一直下降
B.一直上升
C.先升后降
D.先降后升
参考答案:D
Bill wants to buy a computer, but his family is poor. He can't ______ it. [ ]
A. hide
B. sell
C. afford
口腔保健中牙刷刷毛的直径应()
A.<0.5mm
B.<0.4mm
C.<0.3mm
D.<0.2mm
E.<0.1mm