Two concentric shells of masses `M_(1)` and `M_(2)` are having radii `r_(1)` and `r_(2)`. Which of the following is the correct expression for the gravitational field on a mass `m`?
A. `(G(M_(1)+M_(2)))/(r^(2))," for "rltr_(1)`
B. `(GM_(1)+M_(2))/(r^(2))` for `rltr_(2)`
C. `(GM_(2))/(r^(2))` for `r_(1)ltrltr_(2)`
D. `(GM_(1))/(r^(2))` for `r_(1)ltrltr_(2)`