阅读材料 回答题。Once upon a time a poor farmer taking a s

阅读材料，回答题。
Once upon a time a poor farmer taking a sack of wheat to the mill did not know what to do when it slipped from his horse and fell ___51___ the road. The sack was too heavy for him to ___52___, and his only hope was that___ 53___ some one would come riding by and lend a hand.
It was not long before a rider appeared, but the farmer’s heart sank when he ___54___ him, for it was the great man who lived in a castle nearby. The farmer ___55___ have dared to ask another farmer to help, or any poor man who might have come___56___the road, but he could not beg a favor of so great a man.___57___, as soon as the great man came up he got off his horse, saying "I see you’ve had bad luck, friend. How good it is that I’m here just at the___58___ time. "Then he took one___59___ of the sack, the farmer the other, and between them they lifted it on the horse."Sir," asked the farmer, "how can I pay you?"
"Easily enough," the great man___60___ "Whenever you see anyone else in trouble, do the same for him."
__________ 查看材料
A.on
B.in
C.off
D.onto
请帮忙给出正确答案和分析，谢谢！