I'm having a hard time with Redox and how to figure out which is redox

Question

Select all of the reactions that are redox reactions.
I) Ca + 2H2O  Ca(OH)2 + H2
II) CaO + H2O  Ca(OH)2
III) Ca(OH)2 + H3PO4  Ca3(PO4)2 + H2O
IV) Cl2 + 2 KBr  Br2 + 2 KCl

I and II
II and III
I and IV
III and IV

What is the oxidizing agent in the following reaction?
Zn(s) + NO31-(aq)  Zn(OH)42-(aq) + NH3(aq)

Zn(s)
NO31-(aq)
Zn(OH)42-(aq)
NH3(aq)

And no this isnt homework just "practice probs" and no answer...i don't know how to do them or understand them thanx

Answer 1

look at 1) and iv) Both have valences that are different on each side.

Oxidizing agent=reduced species=thing that gained electrons
Zn lost electrons
N went from +5 (In NO3^-1) to +3 in NH3, so N gained electrons.

Answer 2

So then because the valences are diff on each side that means its a redox reaction??

And so N gained eletrons does that mean it's the NO3 ^-1 that is the oxidizing agent or the N in NH3??

this stuff is tripping me up.....can ou explain it any more for me? sorry...im a little slow with chemistry..