He has been studying for many years. He should have realized sooner that his grammar was incorrect.
A.He has been studying for many years to have soon realized that his grammar was incorrect.
B.He has been studying for many years that he should have realized sooner that his grammar was incorrect.
C.No sooner has he been studying for many years than he should have realized that his grammar was incorrect.
D.Though he has been studying for many years, he did not realize sooner that his grammar was incorrect.