Why Does Na2SO4 Get Reduced in This Reaction?

[physicsman2]

Homework Statement

20. Which substance gets reduced in the reaction below?
Na2SO4 + 4 C → Na2S + 4 CO
a.CO
b.Na2SO4
c.C
d.Na2S
e.this is not an oxidation-reduction reaction

Homework Equations

none

The Attempt at a Solution

i know how to find the oxidation numbers, but why is b the answer I don't understand why.
Can anyone explain why? Thanks.

 

[saxen]

Homework Statement

20. Which substance gets reduced in the reaction below?
Na2SO4 + 4 C → Na2S + 4 CO
a.CO
b.Na2SO4
c.C
d.Na2S
e.this is not an oxidation-reduction reaction

Homework Equations

none

The Attempt at a Solution

i know how to find the oxidation numbers, but why is b the answer I don't understand why.
Can anyone explain why? Thanks.

A redox-reaction is when one substance is reduced and one is oxidized. Count all oxidation numbers and see how it changes.