OUR GRANDFATHER _______ AS AN ENGINEER FOR 30 YEARS BEFORE HE RETI...

34. - DE KIEM TRA ANH VAN SO 1HOT
DE KIEM TRA ANH VAN SO 1HOT

34. Our grandfather _______ as an engineer for 30 years before he retired in 1900.

a. worked

b. was working

c. has been working d. had been working