A:when; before
B:since; before
C:after; before
D:before; since
汽车从制动到停止下来共用了5s,这段时间内,汽车每1s前进的距离分别是9m,7m,5m,3m,1m.求汽车前3s和全程的平均速度.
Daming is _____Chinese. He is ____Chinese boy. ( )