The force exerted by a special compression device is given as function of compression x as \[{{F}_{x}}(x)=kx(x-\ell )\] for\[0\le x\le \ell \], where \[\ell \]is maximum possible compression and A: is a constant. The force exerted by the device under compression is maximum when compression is -

• A. 0
• B. \[\ell /4\] 
• C. \[\ell /\sqrt{2}\]
• D. \[\ell /2\]