3、聪聪家承包了村边的小湖养鱼，聪聪为了帮爸爸估算湖里有多少条鱼，从湖里捕上100条做上标记，然后放回湖里，经过一段时间待标记的鱼全混合于鱼群中后，第二次捕得200条，发现其中带标记的鱼25条，那么湖里约有鱼_________条。

The emergency signal for fires is sounded on the ship’s whistle and general alarm as（）.

A.a continuous ringing for 10 seconds

B.one short ring followed by one long ring

C.two long rings of at least 20 seconds

D.a continuous ringing until the fire is extinguished