The potential energy of a particle of mass $5 \mathrm{~kg}$ moving in the $x-y$ plane is given by $U=-7 x+24 y$ joule, $x$ and $y$ being in metre. Initially at $t=0$ the particle is at the origin. $(0,0)$ moving with a velocity of $6[2.4 \hat{\mathrm{i}}+0.7 \hat{\mathrm{j}}]$ $\mathrm{m} / \mathrm{s}$. The magnitude of force on the particle is:
(a) 25 units
(b) 24 units
(c) 7 units
(d) none of these