A thick spherical shell of uniform mass density has an inner radius \(R_1\), an outer radius \(R_2\), and a total mass \(M\). A point particle of mass \(m\) is located at a distance \(r\) from the center of the shell, where \(R_1 < r < R_2\). In terms of the given quantities and the universal gravitational constant \(G\), which of the following expressions represents the magnitude of the net gravitational force exerted by the shell on the particle?