A progressive wave of frequency 500 Hz is travelling with a speed of 330 m/s in air. The distance between the two points which have a phase difference of $30^{\circ}$ is

Question

$\begin {array} {1 1} (1)\;0.11\: m & \quad (2)\;0.055\: m \\ (3)\;0.22\: m & \quad (4)\;0.025\: m \end {array}$