---I heard your grandpa used to be a teacherYes. He was a math teacher .A. on the 19

问题补充：

---I heard your grandpa used to be a teacher.

--- Yes. He was a math teacher .

A. on the 1950s

B. in the 1950s

C. at the 1950’s

D. in 1950’s

答案：

A. on the 1950s

(答案→)B. in the 1950s

C. at the 1950’s

D. in 1950’s

解析：考察年代表达。年份后面加上s，表示某个年代，故选项CD年代表达错误。表示时间时，on表示在具体某一天，in表示在一个时间范围。年代属于一个时间范围，故用介词in，选B。