\textbf{81 --} What is the sum of the stoichiometric coefficients of the substances in the following reaction, after balancing it?
$$\mathrm{CH_4(g) + NH_3(g) + O_2(g) \rightarrow HCN(g) + H_2O(g)}$$
(1) 12 \hfill (2) 13 \hfill (3) 14 \hfill (4) 15
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