通过空气传入广播与电视演播室内的噪声，其路径一般有：门缝、穿线孔洞和（ ）等。
A．电缆
B．墙壁
C．通风管道
D．观察窗
参考答案：C
纯水
A CO2 extinguisher on a ship which has lost 10% of its charge must be（）.
A.used at the earliest opportunity
B.hydro-tested
C.recharged
D.weighed again in one month
