A research vessel is moving directly toward a stationary submarine at a constant speed of 10m s−1. At the instant the distance between them is x, the vessel emits an ultrasonic pulse and receives the echo 3.0s later. Taking the speed of sound in seawater as 1500m s−1, find the initial distance x between the vessel and the submarine when the pulse was emitted.
(a) 2280m
(b) 2265m
(c) 2235m
(d) 2250m