He began to work there ( )

He began to work there ( )
A.on his fifty B.at age of fifty C.when he fifty D.in his fifties

答案D
分析：
A.介词错误.
B修改为：at the age of fifty.在五十岁时
C修改为：when he was fifty.当他五十岁时
D in his fifties在五十多岁时.可以是五十一岁,也可以是五十九岁,有十年的区间.类似的用法有：in his thirties:在他三十多岁时
in 1990s 在二十世纪九十年代