单项选择题

A.间断鸣三次长声,即鸣30s、停、鸣30s、停、鸣30s
B.间断鸣一长、一短三次,即鸣20s、鸣10s、停、鸣20s、鸣10s、停、鸣20s、鸣10s
C.连续三短声,即鸣10s、停、鸣10s、停、鸣10s
D.一次长声,鸣60s