How can identify the substance oxidized

Question

For each reaction:

a.       Provide the oxidation number for each atom

b.      How can identify the substance oxidized and the substance reduced?

c.       Find out the oxidizing agent and the reducing agent.

1. SnO₂ (s) + 2C (s) --> Sn (s) + 2CO (g)

2. 4NH₃ (g) + 5O₂ (g) --> 4NO (g) +6H₂O (I)