Fundamental frequency is given by
$n=\frac{1}{2 l} \sqrt{\frac{T}{\mu}}$
$n \propto \frac{1}{l}(\because T \text { and } \mu \text { are constants })$
Here, $l_{1}=\frac{k}{n_{1}}, l_{2}=\frac{k}{n_{2}}, l_{3}=\frac{k}{n_{3}}$ and $l=\frac{k}{n}$
But $l=l_{1}+l_{2}+l_{3}$
$\therefore \quad \frac{1}{n}=\frac{1}{n_{1}}+\frac{1}{n_{2}}+\frac{1}{n_{3}}$
$(A)$ 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