Answer :
Answer: B. 100 Hz
Explanation:
Here, the frequency for the standing-wave modes of a tube is given as
f(n)= m*f
For first frequency put f= 20 Hz, m=1
So f₁= 1*20= 20Hz
The next resonance occurs at m=3
f₂= 3*20= 60 Hz
So meaning at odd intervals the resonance occurs so next will occur at m=5
So,
f₃= 5*20= 100 Hz
So, option B. 100 Hz is correct.