${n_1}{l_1} = {n_2}{l_2} = {n_3}{l_3}..... = $constant $= k$ (say) $= nl$
Also ${l_1} + {l_2} + {l_3} + {l_4} + ...... = 1$
$\frac{k}{{{n_1}}} + \frac{k}{{{n_2}}} + \frac{k}{{{n_3}}} + \frac{k}{{{n_4}}} + .... = \frac{k}{n}$==> $\frac{1}{n} = \frac{1}{{{n_1}}} + \frac{1}{{{n_2}}} + \frac{1}{{{n_3}}} + .......$

$(A)$ the intensity of the sound heard at the first resonance was more than that at the second resonance
$(B)$ the prongs of the tuning fork were kept in a horizontal plane above the resonance tube
$(C)$ the amplitude of vibration of the ends of the prongs is typically around $1 \mathrm{~cm}$
$(D)$ the length of the air-column at the first resonance was somewhat shorter than $1 / 4$ th of the wavelength of the sound in air
$(A)$ With a node at $O$, the minimum frequency of vibration of the composite string is $v_0$
$(B)$ With an antinode at $O$, the minimum frequency of vibration of the composite string is $2 v_0$
$(C)$ When the composite string vibrates at the minimum frequency with a node at $O$, it has $6$ nodes, including the end nodes
$(D)$ No vibrational mode with an antinode at $O$ is possible for the composite string
