1. In a closed organ pipe, standing waves can be formed.
II. In a closed organ pipe (closed at one end), only odd harmonics are present.
III. The harmonics which are present in a pipe, open at both ends are odd harmonics only.
Which of the following statement(s) related to organ pipe is/are correct?

In a closed organ pipe, two waves travelling in opposite direction superimpose with each other to develop a wave pattern which is standing or stationary.

Possible wavelengths of stationary waves in this type of organ pipe is given as

${\lambda }_n=\frac{4L}{(2n+1)}\text{ or }{\lambda }_n=4L,\frac{4L}{3},\frac{4L}{5},\dots \dots$

As frequency, $v=\frac{v}{\lambda }$   (On putting, n = 0, 1, 2, 3, …)

So, frequencies of nth harmonics can be given as

$v_n=\frac{v}{4L},\frac{3v}{4L},\frac{5v}{4L},\dots$

$\therefore v_1:v_2:v_3\dots \dots =1:3:5$

So, only odd harmonics are present in case of closed organ pipe.
However, in case of open organ pipe,

frequencies of nth harmonics,  $v_n=\frac{nv}{2L}$

Thus, even and odd or all the harmonics are present. 

So, Statements I and II are correct but III is incorrect.