什么叫CO2焊接？
参考答案：用纯度>99.98%的CO2做保护气体的熔化极气体保护焊—称为CO2焊。
After the quarrel, I tried all my best to fix the misunderstanding between us, but all my efforts seemed _____. [ ]
A. in vain
B. in case
C. in time
D. in trouble
心理分析派小说最主要的代表作家是（）。
