A parallel plate capacitor having cross-sectional area 'A' and separated by distance 'd' is filled by copper plate of thickness b. It's capacitance is :
1
\(\frac{\varepsilon_0 A}{2d}\)
2
\(\frac{\varepsilon_0 A}{d-b}\)
3
\(\frac{2\varepsilon_0 A}{d+\frac{b}{2}}\)
4
\(\frac{\varepsilon_0 A}{d+\frac{b}{2}}\)