He arrived in class. He realized he had forgotten his book on the bus.
A.No sooner had he realized he had forgotten his book on the bus than he arrived in class.
B.Had he arrived in class, he would have realized he had forgotten his book on the bus.
C.Only after he had realized he had forgotten his book on the bus did he arrive in class.
D.Not until he arrived in class did he realize he had forgotten his book on the bus.