Two concentric shells of masses $M_1$ and $M_2$ are having radii $r_1$ and $r_2\mathit{.}$ Which of the following is the correct expression for the gravitational field at a distance $r.$

A solid sphere of radius $R/2$ is cut out of a solid sphere of radius $R$ such that the spherical cavity so formed touches the surface on one side and the centre of the sphere on the other side, as shown. The initial mass of the solid sphere was $M$. If a particle of mass $m$ is placed at a distance $2.5 R$ from the centre of the cavity, then what is the gravitational attraction on the mass $m$ ?

A solid sphere of uniform density and radius $R$ applies a gravitational force of attraction equal to $F_1$ un a particle placed at a distance $2R$ from the centre of the sphere. A spherical cavity of radius $R/2$ is now made in the sphere as shown in the figure. The sphere with the cavity now applies a gravitational force $F_2$ on the same particle. The ratio $F_1/F_2$ is