钛及钛合金管焊接应采用( )进行焊接。
A．二氧化碳气体保护焊
B．惰性气体保护焊
C．氧-乙炔焊
D．真空焊
参考答案：B,D
We were delayed at the airport．Otherwise，we here by lunch．
A．must have been
B．could be
C．should be
D．would have been
“十月怀胎，一朝分娩”．母亲与胎儿之间进行物质和气体交换主要发生在（ ）
A．子宫
B．输卵管
C．胎盘
D．脐带
