The force exerted by a compression device is given by \[F(x)=kx\,(x-l)\,for\,0\le x\le l\], where \[l\] is the maximum possible compression, \[x\] is the compression and is a constant. The work required to compress the device by a distance \[d\]will be maximum when:

• A. \[d=\frac{l}{4}\]         
• B. \[d=\frac{l}{\sqrt{2}}\]
• C. \[d=\frac{l}{2}\]         
• D. \[d=l\]