There are two walls A and B which are 180 m apart from each other. A boy standing at 60 m away from wall A claps his hands once. If the speed of sound in air is 330 m s-1, what is the time interval between the first and the second echo that he hears?
A 0.36 s
B 0.24 s
C 0.4 s
D 1 s