He must have had an accident , or he        here for class then .

A．had to be

B．should be

C．would be

D．would have been

核酸类型为DNA的病毒是（）

A.流感病毒

B.副流感病毒

C.呼吸道合胞病毒

D.腺病毒

E.麻疹病毒