1 when he was ______,he began to learn computer.

1 when he was ______,he began to learn computer.
A at his fifties B in his fifty C in his fifties D on his fiftieth
2 Do you know the______,"Time lost can't be won again''(say)
3 I don't know what I shall do.（改为简单句） I don't know what _______ _________.
4 They set up a new great bridge over the Huangpu River last year.(改为被动语态） A new great bridge over the Huangpu River _______ _________ _________ last year.
5 medical的名词＿＿＿＿＿
6 a _______(wood) table
请写出各题原因