明挖爆破时，准备信号是（ ）。
鸣30s、停、鸣30s、停、鸣30s
鸣20s、鸣10s、停、鸣20s、鸣10s、停、呜20s、鸣10s
鸣10s、停、鸣10s、停、鸣10s
鸣20s、停、鸣20s、停、呜20s
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