Determine the oxidation numbers of each of the elements in the following reactions, and identify which of them are oxidized or reduced, if any.
a. SiO₂ (s) + 2 H₂O(l) → HĄSiO₄(aq)
b. 2 MnCO₃(s) + O₂(g) → 2 MnO₂(s) + 2 CO₂(g)
c. 3 NO₂(g) + H₂O(C) → 2 NO3- (aq) + NO(g) + 2 H+(aq)