Two planets A and B of radii \( R \) and \( 1.5R \) have densities \( \rho \) and \( \dfrac{\rho}{2} \) respectively. The ratio of acceleration due to gravity at the surface of B to A is:
\( 2:3 \)
\( 4:3 \)
\( 3:4 \)
\( 2:1 \)