A:①
B:②
C:①③
D:②③
一矿井深45米,在井口每隔一定时间自由落下一个小球,当第6个小球从井口开始下落时,第一个小球恰好落至井底(g=10m/s2)问:
Be careful! In front of our bikes ____ (lie) a big black dog.
I ____ (write)a letter to my friend yesterday.