A student sets up the speed of sound in air apparatus for finding the velocity of sound in air, at the room temperature. Using a tuning fork, of frequency n (Hz), she obtains the first and second positions of resonance for lengths l1 (m) and l2 (m) , respectively, of the air column, in the resonance tube. The velocity of sound, V(in m/s), would then be best given by the formula.
(1) V = 4l1n
(2) V = 4/3 l2n
(3) V = 2(l2 - l1)n
(4) V = (l2 + l1)n