  We establish a characterization for an $m$-manifold $M$ to admit $n$
functions $f_1$,...,$f_n$ and $n'$ functions $g_1,...,g_{n'}$ in
$\mathcal{C}^\infty(M)$ so that every element of $\mathcal{C}^k(M)$ can be
approximated by rational combinations of $f_1,...,f_n$ and polynomial
combinations of $g_1,...,g_{n'}$. As an application, we show that the optimal
value of $n$ and $n'$ for all manifolds of dimension $m$ is [3m/2], when $k\geq
1$ and $m\geq 2$.
