The Muskingum model of routing a flood through a stream reach is expressed as $O_2 =K_0I_2+K_1I_1+K_2O_1$, where $K_0$, $K_1$ and $K_2$ are the routing coefficients for the concerned reach, $I_1$ and $I_2$ are the inflows to the reach, and $O_1$ and $O_2$ are the outflows from the reach corresponding to time steps $1$ and $2$ respectively. The sum of $K_0$, $K_1$ and $K_2$ of the model is

1. $-1$
2. $-0.5$
3. $0.5$
4. $1$