明挖爆破时，预告信号是（）。

A.鸣30s.停.鸣30s.停.鸣30s

B.鸣20s.鸣10s.停.鸣20s.鸣10s.停.鸣20s.鸣10s

C.鸣10s.停.呜10s.停.鸣10s

D.鸣20s.停.鸣20s.停.鸣20s