For p-dichlorobenzene and p-cyanobenzene. the dipole moment of individual bonds cancel each other as they are equal in magnitude and opposite in direction. Hence, resultant dipole moment of molecules is zero. For p-hydroquinone and p-benzenedithiol,the O−H and S−H bond dipoles in these molecules do not cancel each other as they are not in opposite directions due to existence in different conformation.