单项选择题

Once upon a time a poor farmer taking a sack of wheat to the mill did not know 1 to do when it slipped from his horse and fell onto the road. The sack was 2 heavy for him to lift, and his only hope was that 3 some one would come riding by and lend a hand.
It was not long 4 a rider appeared, but the farmer"s heart sank when he recognized him, for it was the great man who lived in a castle nearby. The farmer 5 have dared to ask another farmer to help, or any poor man who might have come 6 the road, but he could not beg a favor of so great a man. 7 , 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 8 I"m here just at the right time". Then he took one 9 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 replied. "Whenever you see 10 else in trouble, do the same for him."

A.when
B.which
C.then
D.that