单项选择题

Nasreddin was an old man. One day he and some of his old friends were talking (41) young people in their town. They (42) agreed that old people were cleverer than (43) . Then one of the old men said, "But young men are stronger than old men." (44) of them agreed that it was fight except (除…以外) Nasreddin. He said, "No, I am as strong now as I (45) in my young days." "What do you mean" said his friends. "How (46) that be true" "Well," said Nasreddin, "There is a big stone at a comer of my field. (47) I was a young man, I tried to (48) it, but I couldn’t. I am old now. I (49) can’t move it, just like when I was (50) .\