The mass density of a spherical body is given by $\rho ( r ) = \frac { k } { r }$ for $r \leq R$ and $\rho ( r ) = 0$ for $r > R$, where $r$ is the distance from the center. The correct graph that describes qualitatively the acceleration, $a$ of a test particle as a function of $r$ is: (options given as graphs (1), (2), (3), (4))