铸铁件常用的焊修方法，按焊件预热的程度不同，可分为（）
A、热焊
B、冷焊
C、快焊
D、慢焊
参考答案：A, B
命题“∃x∈R，使得x2＞0”的否定是______．
A．It was great. B．what’s the matter C．How about the green one D．Are you free tomorrow E．Thanks, but I am full.
A: So many nice coats! I can’t decide which one to buy. B: ______
