$C_0$ 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. $c_0(1+\epsilon _r)$

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