压力容器法兰垫片按材质和结构不同分为哪三类？
参考答案：
1、金属包垫片；
2、金属软垫片；
3、缠绕式垫片。
He said he would pick me up at the school gate. However, he ______ yet. [ ]
A. didn't arrive
B. doesn't arrive
C. isn' t arriving
D. hasn't arrived
当井斜角大约为多少时，仪器下井过程中自身重量小于或等于各种阻力之和？
