A mass $m$ is at a distance $a$ from one end of a uniform rod of lenght $l$ and mass $M$. The gravitational force on the mass due to the rod is
(a) $\frac{G M m}{(a+l)^{2}}$
(b) $\frac{G m M}{a(l+a)}$
(c) $\frac{G M m}{a^{2}}$
(d) $\frac{G m M}{2(l+a)^{2}}$