在焊前装配时,先将焊件方向于焊接变形相反的方向进行人为的变形,称为反变形法()
参考答案:对
紧急停车时,精馏系统()。
A.不必保持压力
B.保持压力
C.两可
—Catherine, I have cleaned the room for you.
—Thanks. You _____it. I could manage it myself.
A.needn’t do
B.needn’t have done
C.mustn’t do
D.shouldn’t have done