The heat of combustion of methane, carbon monoxide and hydrogen are $P$, $Q$ and $R$ respectively. For the reaction below, $$CH_4+H_2O \rightarrow CO+3H_2$$ the heat of reaction is given by

1. $P- Q-3R$
2. $Q+3R-P$
3. $P-Q-R$
4. $Q+R-P$