[" 48.Charges are placed on the vertices of a square as "],[" shown.Let "vec E" be the electric field and V the "],[" potential at the centre.If the charges on A and B "],[" are interchanged with those on "D" and "C],[" respectively,then "],[qquad -q(1)/(E)" main unchanged "],[" (b) "vec E" remains,V remains unchanged "],[" (b) "vec E" remains unchanged,V changes "],[" (d) Both "vec E" and "V" change "]