Listen carefully, _______ you can’t hear what the teacher is saying.

A．and

B．then

C．or

D．but

螺旋桨结冰会：（）.

A.减小拉力并产生振动

B.增大拉力但产生过大振动

C.增加失速转速和噪音

D.降低失速转速和噪音