He learned well enough to play the piano to the public when he was ________.

He learned well enough to play the piano to the public when he was ________.

A.at the age of 12 years old

B.a twelve years old boy

C.a 12-years old boy

D.a twelve-year-old boy

答案及解析： D

解析：

导解：year在此结构中不用复数．