A solid sphere of radius (R) has total charge (2)(Q) and volume charge density (p)=(k)(r) where (r) is distance from centre. Now charges (Q) and -(Q) are placed diametrically opposite at distance (2)(a) where a is distance form centre of sphere such that net force on charge (Q) is zero then relation between a and (R) is (A) (a)=(R)/(2) (B) (a)=(R) (C) (a)=(2)(R) (D) (a)=(3)(R)/(4)