设A、B为随机事件，P（A）=0.7，P（A-B）=0.3，则=（）。
A．0.3
B．0.4
C．0.6
D．0.7
参考答案：C
解析：
—I failed the exam again. I don't know what to do.
—_____ Hope is always around you. [ ]
A. Don't be serious
B. Never give up
C. Don't be relaxed
D. Never put down
火力发电厂（）管道上装设的测温元件，宜有耐磨的保护管。
A、一、二次风
B、烟气
C、气粉
=（）。
=（）。