在常量分析时，载气、氢气和空气的纯度在( )以上即可。
A．99.9%
B．99.999%
C．95%
D．90%
参考答案：A
It isn't going to hurt, ____________ it?[ ]
A. does
B. isn't
C. is
D. doesn't
下列去括号正确的是 [ ]
A．a﹣（2b﹣c）=a﹣2b﹣c
B．﹣（m+n）+（x﹣y）=﹣m﹣n+x﹣y
C．（2m+n）﹣（p﹣1）=2m+n+p﹣1
D．a﹣（3x﹣y+z）=a﹣3x﹣y﹣z
