PLEASE HELP ME ASAP question: which of the following represents a polar covalent compound?

choices: a. O2 b. NaCl c. Cl2 d. H2O

all i know is that it isn’t b. NaCl if that helps

Carbon is allowed to diffuse through a steel plate 15 mm thick. the concentrations of carbon at the two faces are 0.65 and 0.30 kg c/m^3 fe, which are maintained constant. if the preexponential and activation energy are 6.2 x 10-7 m2 /s and 80,000 j/mol, respectively, compute the temperature at which the diffusion flux is 1.43 x 10^-9 kg/m^2 -s.