正压式空气呼吸器佩戴时必须首先确保面罩气密性再打开呼吸阀控制开关。
参考答案:对
在等差数列{an}中,a5+a6+a7+a8+a9=450,则a3+a11的值为( )
A.45
B.75
C.180
D.300
I telephoned Jane just now, but she wasn't in. I has to leave her a(n) _______. [ ]
A. advice
B. information
C. message
D. paper