The speed of sound in sea water is 1490m/s at 25∘C and 1530m/s at 35∘C. A submarine records an echo time of 1.0s at 35∘C. By how many metres would the calculated depth differ from a calculation made at 25∘C using the same time delay?
(a) 40m deeper
(b) 20m shallower
(c) 20m deeper
(d) No difference