A solid insulating sphere of radius \(R\) has a uniform volume charge density \(\rho_0\). A spherical cavity of radius \(b\) (where \(b < R\)) is removed from the sphere, with its center located at position vector \(\vec{a}\) relative to the sphere's center \(O\). Point \(P\) is located inside the cavity at position vector \(\vec{r}\) relative to center \(O\). Which of the following expressions correctly gives the electric field vector \(\vec{E}\) at point \(P\) inside the cavity?