$2.$ A $500-$kg cannon fires a $4\mathrm{.}0-$kg projectile horizontally with a velocity of $500$ m$/$s

relative to the ground. If the cannon is able to move freely, what is the recoil speed of

the cannon?

A$)2\mathrm{.}0$ m$/$s

B$)8\mathrm{.}0$ m$/$s

C$)6\mathrm{.}0$ m$/$s

D$)10$ m$/$s

E$)4\mathrm{.}0$ m$/$s