An open pipe of sufficient length is dipping in water with a speed v vertically. If at any instant $l$  is length of tube above water, then the rate at which fundamental frequency of pipe changes, is(speed of sound =c) 

• A

  $\mathrm{cv}/2{\mathrm{l}}^2$

• B

  $\mathrm{cv}/4{\mathrm{l}}^2$

• C

  $\mathrm{c}/2{\mathrm{v}}^2{\mathrm{l}}^2$

• D

  $\mathrm{c}/4{\mathrm{v}}^2{\mathrm{l}}^2$