(Any number of options may be correct.) Three concentric spherical shells have radii r, 2r and 3r with charges q1, q2 and q3 respectively. Innermost and outermost shells are earthed. Then, (a) q1+q3=-q2 (b) q1=-q2/4 (c) q3/q1=3 (d) q3/q2=-1/3 The answer key tells me that (a), (b), (c) are correct. Could anyone please help me solve this?