一般地说,在远场同一深度的平底孔直径增大一倍,其回波声压提高()。
A、1倍
B、9dB
C、12dB
D、24dB
参考答案:C
It’s ________ for us to eat too ________ meat.
A.healthy; many
B.unhealthy; many
C.healthy; much
D.unhealthy; much
氮气系统有何作用?