A dielectric slab with $500$ $mm$ $×$ $500$ $mm$ cross-section is $0.4$ $m$ long. The slab is subjected to a uniform electric field of $E=6a_x+8a_y$ $KV/mm$. The relative permittivity of the dielectric material is equal to $2$. The value of constant $\varepsilon _0$  is $8.85*10^{-12} F/M$ . The energy stored in the dielectric in Joules is
1. $8.85*10^{-11}$
2. $8.85*10^{-15}$
3. $88.5$
4. $885$