一道完形填空题,高手帮我填,并给我分析为什么是这样子填的.

一道完形填空题,高手帮我填,并给我分析为什么是这样子填的.
不要仅给答案要给我分析.不会的离开.
One day a snake was sleeping in the sun_1_the foot of a mountain.___a big stone ___down the mountain and it fell on the top the snake.The stone was so___ that the snake could not move.Just then a hare came by.He saw the snake___under the store,so he tried___to help the snake out of___.But the snake was ___bad that he wanted to eat the hare.When the clever deer knew what___.He thought for a few seconds and had___idea."Tell me___.Mr Snake."Said the deer."Then I"ll tell who is right and who is wrong."The hare and the deer put the stone back onto the snake and the snake was___again.The clever deer saved the hare,and the snake had to stay under the stoon for ever,for the could not move,
（ ）A.on B.under C.at D.over
( )A.suddenly B.luckily C.happily D.fortunately
( )A.fall B.falling C.fallen D.fell
( )A.high b.lighr C.narrow D.heavy
( )A.lying B.laying C.lay D.lain
( )A.better B.best C.his best D.his better