In which of the following transformations does the nitrogen atom NOT undergo reduction?
NO3−→NH4+\text{NO}_3^- \rightarrow \text{NH}_4^+NO3−→NH4+
2NO2→N2O42\text{NO}_2 \rightarrow \text{N}_2\text{O}_42NO2→N2O4
N2→2NH3\text{N}_2 \rightarrow 2\text{NH}_3N2→2NH3
HNO3→NO\text{HNO}_3 \rightarrow \text{NO}HNO3→NO