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The length of two open organ pipes are l and $\left(l+\Delta l\right)$ respectively. Neglecting end correction, the frequency of beats between them will be approximately

(1) $\frac{v}{2l}$

(2) $\frac{v}{4l}$

(3) $\frac{v\Delta l}{2{l}^{2}}$

(4) $\frac{v\Delta l}{l}$

(Here v is the speed of sound)