氧气切割时，预热火焰的作用是（）。
A.加热
B.加热到燃点
C.加热到熔点
参考答案：B
As the final examination is coming , the students are busy ____ it.
A． preparing
B． preparing for
C．prepare
D．prepare for
纵隔向健侧移位见于()
A．张力性气胸
B．肺不张
C．蜂窝肺
D．一侧肺切除
E．大叶性肺炎
