国家规定的天然气灶具的使用年限是（）年
A.6
B.8
C.10
参考答案：B
Had the bomb dropped over a populated area of the city , there ____ a great deal of damage．
A．may be
B．might have been
C．may not be
D．mightn’t have been
同直径的平底孔在球面波声场中距离增大一倍，则回波高度降低（）。
A.6dB
B.12dB
C.3dB
D.9dB
