开大原料预热炉火嘴的风门，燃烧空气量会增大，炉膛负压会（）
A、降低
B、升高
C、不变
D、无法确定
参考答案：A
Just after putting away the dishes, _______.
A．the doorbell was rung
B．mother heard the doorbell ring
C．someone knocked at the door
D．the doorbell rang loud
试述螺杆泵振动的原因及排除方法？
