28.Four point charges 1 `mu` C -2 `mu` C 3 `mu` C and -2 `mu` C are arranged on the four vertices of a square of side 1cm .The dipole moment of this charge assembly is 1) zero 2) `sqrt(2)times10^(-8)C.m` 3) `2sqrt(2)times10^(-8)C m` 4) `2times10^(-8)C m`