At 20∘C, a student standing at a distance d from a vertical cliff claps and hears the echo after 2.4s. Two weeks later, when the temperature is 40∘C, he moves 60m closer to the cliff and now hears the echo after 2.0s. Taking the temperature dependence of sound speed into account, calculate the original distance d (in m).
(a) 400
(b) 433
(c) 412
(d) 500