问题补充：
Pierre began to help his father with the farm work when he was
A. In 1980’s; his teens
B. At the 1980’s; the teens
C. In the 1980s; in his teens
D. In 1980’s; in the teens
答案：
A. In 1980’s; his teens
B. At the 1980’s; the teens
(答案→)C. In the 1980s; in his teens
D. In 1980’s; in the teens
解析：“在二十世纪八十年代” In the 1980s，“在某人几十多岁时”in sb’+岁数名词的复数，所以答案C。
