(A) CH4 is a nonpolar molecule because the bonds between carbon and the four hydrogen atoms are polar.
(B) O2 is a polar molecule because the pull on electrons shared between the oxygen atoms is identical.
(C) CO2 is a nonpolar molecule because oxygen exerts greater pull on the shared electrons than does carbon.
(D) H2O is a polar molecule because part of the molecule is partially positively charged while the remaining part is partially negatively charged.