His father __________ have a ball.
A:doesn't
B:don't
C:isn't
D:aren't
将一个小球从报废的矿井口由静止释放后做自由落体运动,4s末落到井底。该小球开始下落后第2s内和第4s内的平均速度之比是 ( )
在超市买樱桃时,售货员阿姨对顾客说:您的樱桃是0.5千克.”
0.5千克合____ 千克.(化成分数)