The girl hurt her leg because she ____ her bike.A:fell off B:fell down

是选A,两个词组一个相当于及物动词 ,另外一个相当于不及物动词
fall down,fall off和fall over都表示“摔倒、跌倒”的意思,但它们各自的含义不同.
fall over强调的是“向前摔倒、跌倒”.
She fell over and broke her leg.
There is too much snow on the ground.If you ride much too fast,you will fall over.
fall down强调的是“滑倒、倒下”,后接宾语时应加上介词from
The boy hit the tree to hard that he fell down.
If people run across the road,they may fall down.
fall off
强调的是“跌落、从……掉下来”,后直接接宾语.
The girl is falling off the bike.（= The girl is falling down from the bike.）