textbook 9C.5

[005532550]

The entirety of number five here confused me. After rereading the textbook and some of the responses on chemistry community, determining whther a compound is polydentate make sense but I still am unsure how to determine the number of binding sites a ligand can have. Would someone be able to explain the reason behind the correct answers for #5? The question is:
Which of the following ligands can be polydentate? If the ligand can be polydentate, give the maximum number of places on the ligand that can bind simultaneously to a single metal center:
(a)HN(CH2CH2NH2)2
(b)CO3^2-
(c)H2O
(d) oxalate.

thanks you!

[Natalie Keung 1D]

a)3 binding sites since the molecule has 3 Nitrogen atoms. Each N has a lone pair that can allow the ligand to bond to the ligand center. This can be a tridentate ligand.
b)2 binding sites to the two charged Oxygen atoms. The ligand can either be binded with one Oxygen atom or both so it is monodentate or bidentate.
c)H2O is a known monodentate ligand. (Ligands with one charge (-1) or are neutral tend to be monodentate)
d)2 binding sites to the charged oxygens. This makes it a bidentate ligand.

Whenever I encounter a ligand with charged O, it tends to be ___dentate according to the total ion charge. I am not sure if this is proper, but it's just how I've been approaching these types of questions.