Statement I: Benzene has zero dipolemoment whereas chlorobenzene has non zero dipole moment.
Statement II : All symmetrical molecules have zero dipole moment & it is non zero for unsymmetrical molecules.

In benzene all carbon atom are sp² hybridized.
It is a planar symmetrical molecule with zero dipole moment
 

has polar C-Cl bond
 

μ ≠ 0
Polar unsymmetrical molecule
All symmetrical molecules  = μ = 0
All unsymmetrical molecules = μ ≠ 0