The figure represents two concentric shells of radii `R_(1)` and `R_(2)` and masses `M_(1)` and `M_(2)` respectively. The gravitational field intensity at the point `A` at distance `a (R_(1) lt a lt R_(2))` is
A. `(G(M_(1) + M_(2)))/(a^(2))`
B. `(GM_(1))/(a^(2)) + (GM_(2))/(R_(2)^(2))`
C. `(GM_(1))/(a^(2))`
D. zero