对干气密封的结构，下面理解正确的是（）。
A.其内部有压力降
B.内部有梳齿
C.是机械密封
D.有内腔外腔之分
参考答案：A, B, D
Scott was very nervous at the beginning, but finally he _____ his courage to speak it out, and the teacher accepted his apology and forgave him.
A．made up
B．brought up
C．worked up
D．lived up
热粘合有哪些？
