Suggest a scheme of classification of the following redox reactions:
$(a) N_{2(g)} + O_{2(g)} \rightarrow 2NO_{(g)}$
$(b) 2Pb(NO_3)_{2(s)} \rightarrow 2PbO_{(s)} + 4NO_{2(g)} + O_{2(g)}$
$(c) NaH_{(s)} + H_2O_{(l)} \rightarrow NaOH_{(aq)} + H_{2(g)}$
$(d) 2NO_{2(g)} + 2OH^{-}_{(aq)} \rightarrow NO_2^-(aq) + NO_3^-(aq) + H_2O_{(l)}$

(N/A) In reaction $(a)$,the compound nitric oxide is formed by the combination of elemental substances,nitrogen and oxygen; therefore,this is a combination redox reaction.
In reaction $(b)$,lead nitrate breaks down into three components; therefore,this is a decomposition redox reaction.
In reaction $(c)$,the hydrogen of water is displaced by the hydride ion to form dihydrogen gas; therefore,this is a displacement redox reaction.
In reaction $(d)$,$NO_2$ ($+4$ oxidation state) disproportionates into $NO_2^-$ ($+3$ oxidation state) and $NO_3^-$ ($+5$ oxidation state); therefore,this is a disproportionation redox reaction.