John was fired by his factory and only then ________ how wrong he was to be lazy and careless.[ ]

A. had he realized  

B. he had realized

C. did he realize  

D. he realized

从1998年起，我国实行无偿献血制度．在给病人输血时，输血的原则是（　　）

A．输同型血

B．输A型血

C．输B型血

D．输O型血