明挖爆破时，准备信号是（  ）。

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、鸣10s、停、鸣10s

D 鸣20s、停、鸣20s、停、鸣20s