218)should be (A).
Use the concept that if a charge Q is kept in a hollow sphere at any pos.,the field externally is uniform,and equal to the field caused if charge was at centre.
13)(a)(b)(c)(d)
Pressure=normal force per unit area.
In B net force=wt+centrifugal force
in C net force=wt
in A net force=wt-centrifugal force
