After 4 years abroad, he returned home as an excellent engineer.
A.After he studied 4 years abroad, he returned home as an excellent engineer.
B.After he has studied for 4 years abroad, he returned home as an excellent engineer.
C.After he studied 4 years abroad, he returned home as an excellent engineer.
D.After he had studied for 4 years abroad, he returned home as an excellent engineer.