$\therefore \text { A and } B \text { are wrong. }$
$\mathrm{v}_{\text {sound }}=\sqrt{\frac{\gamma \mathrm{RT}}{\mathrm{M}}}=\sqrt{\left(\frac{2}{\mathrm{f}}+1\right) \frac{\mathrm{RT}}{\mathrm{M}}}$
$\Rightarrow \text { more 'f', less 'v' }$
$\therefore \mathrm{v}_5>\mathrm{v}_7$

