An iceberg is 3,000 metres from a submarine. The submarine emits ultrasound. How long until the submarine detects the iceberg?

3.92s
Ultrasound works by the waves traveling to the object, bouncing off, then returning. So the distance traveled is twice that of the actual distance.
Therefore d=6,000m
d= speed x time
Speed of sound in water is about 1,530.
6000 = 1530 * t
t = 3.92s

Ibelieve your answer is b.hope this : )