A pipe closed at one end and open at the other end resonates with a sound of frequency 135 Hz and also with 165 Hz, but not at any other frequency intermediate between these two. Then, the frequency of the fundamental note of the pipe is

Question

$$\begin {array} {1 1} (1)\;15\: Hz & \quad (2)\;60\: Hz \\ (3)\;7.5\: Hz & \quad (4)\;30\: Hz \end {array}$$