$C_0$ is the capacitance of a parallel plate capacitor with air as dielectric $($ as in figure $(a))$. If, half of the entire gap as shown in figure $(b)$ is filled with a dielectric of permittivity $\epsilon _r$ , the expression for the modified capacitance is
1. $\frac{C_0}{2}(1+\epsilon _r)$
2. $(C_0+\epsilon _r)$
3. $\frac{C_0}{2}\epsilon _r$
4. $C_0(1+\epsilon _r)$