Which of the following is non-polar?

Which one of the following has the highest dipole moment?
$(i)$ $CH_2Cl_2$
$(ii)$ $CHCl_3$
$(iii)$ $CCl_4$

For which molecules among the following,the resultant dipole moment $(\mu) \neq 0?$

Which one of the following molecules has a zero dipole moment?

The critical temperature of water is higher than that of $O_2$ because $H_2O$ molecules ......

If an $AB_3$ molecule (containing four atoms) has a dipole moment of zero and another has a non-zero dipole moment,what does this indicate about their shapes?