CHEMISTRY QUESTION! REDOX REACTIONS! PLEASE HELP ME NOW :C

85. Given the balanced equation representing a reaction: 2KClO3(s) -->
2KCl(s) + 3O2(g)
The oxidation state of chlorine in this reaction changes from
a.
-1 to +1
c.
+1 to -1
b.
-1 to +5
d.
+5 to -1


AND

84. Which equation represents an oxidation-reduction reaction?
a.
Na + Cl2 >> NaCl
b.
H2SO4 + Ca(OH)2>>>>
CaSO4 + 2H2O
c.
MgCrO4 + BaCl2>>>>
MgCl2 + BaCrO4
d.
Zn(NO3)2 +Na2CO3>>>
2NaNO3 + ZnCO3

(For 84. I knew it was A. But just never knew why. Please explain this and 85 for me)
I have a chem test tomorrow :C

Thank you so much
You would save my life <3

85. In KClO3, the K is +1 and the O total is -6 (each O is -2), so the Cl in KClO3 is +5. In KCl, the K is +1 so the Cl is -1. Answer d.

84. Answer a is the only one in which oxidation states change. The Na goes from 0 to +1 and the Cl2 goes from 0 to -1. In all the other substances in each reaction there are no changes of oxidation number.

85. Given the balanced equation representing a reaction:
2KClO3(s) --> 2KCl(s) + 3O2(g)
The sum of the charges in formula = 0
In KClO3, K = +1, O = -2
+1 + Cl + (3 * -2) = 0
+1 + Cl + -6 = 0
Cl = +5

In KCl, K = +1, Cl = -1

The oxidation state of chlorine in this reaction changes from
a.
-1 to +1
c.
+1 to -1
b.
-1 to +5
d.
+5 to -1 YES


AND

84. Which equation represents an oxidation-reduction reaction?
The charge of one element must increase = oxidation
and
The charge of another element must decrease = reduction
Determine the charge by formula rule, which I used above!
For an element, charge = 0
Sum of charges in ion = charge of ion
a.
Na + Cl2 >> NaCl
Na, 0 → +1
Cl, 0 → -1
Redox

b.
H2SO4 + Ca(OH)2>>>> CaSO4 + 2H2O
Same ions on both sides! Ions switch places, but no change of ox number (charge)
Double replacement
c.
MgCrO4 + BaCl2>>>> MgCl2 + BaCrO4
same as above

d.
Zn(NO3)2 +Na2CO3>>> 2NaNO3 + ZnCO3
same as above

(For 84. I knew it was A. But just never knew why. Please explain this and 85 for me)
I have a chem test tomorrow :C