电子束焊设备应装置专用地线，且接地电阻应小于（）Ω。
A.3
B.4
C.5
参考答案：A
解方程：（1）x2+5x+6=0 （2）x（x+5）=x+5．
The weather turned out to be fine yesterday. I __________ the trouble to carry my umbrella with me.
A．should have taken
B．could have taken
C．needn’t have taken
D．mustn’t have taken
