--Mum, I don’t think I am qualified enough to do this. --Honey, be confident! You should know it is ____ a man thinks of himself ___ really determines his fate.
—This returned scholar has become one of the top experts in this field.
—Yes, I know him very well. He ______ for ten years at an institute in the USA.
A. has worked B. has been working C. worked D. had worked