The correct option is C 3t
Velocity of sound, v=√γRTM
where γ is the adiabatic index, R is universal gas constant, T is absolute temperature and M is molecular mass of the gas.
At the same temperature,
⇒vO2vHe=√γO2γHe×MHeMO2
⇒vO2vHe=
⎷(75)(53)×432
⇒vO2vHe=√2125×432
⇒vO2=0.32 vHe ....(1)
Time taken by sound in Helium gas to cover distance l,
t=lvHe .....(2)
Now, the time taken by sound in Oxygen gas to cover distance l,
t′=lvO2
⇒t′=l0.32×vHe [from (1)]
⇒t′=t0.32 [from (2)]
⇒t′≈3t