Which of the following can function as a chelating agent? And Why?

a. NH3

b. CO2

c. OH-

d. C2O4 2-

In oxalate, you need have to have two donor atoms to make a chelate ligand which barely means, NH3 and OH^ - is ruled out. You have to consider the structure of Carbon dioxide (CO2) and deduce why both O atoms cannot coordinate to the same M^n + ion.