Two Parallel Circular Plates, Each with a Radius Of $7.0 \mathrm {~mm}$

Two parallel circular plates, each with a radius of $7.0 \mathrm {~mm}$ and carrying equal-magnitude surface charge densities of $\pm 3.0 \mu \mathrm { C } / \mathrm { m } ^ { 2 }$ , are separated by a distance of $1.0 \mathrm {~mm}$ with only air between them. How much energy is stored in these plates? $\left( \varepsilon _ { 0 } = 8.85 \times 10 ^ { - 12 } \mathrm { C } ^ { 2 } / \mathrm { N } \cdot \mathrm { m } ^ { 2 } \right)$

A) $250 \mathrm {~nJ}$
B) $25 \mathrm {~nJ}$
C) $78 \mathrm {~nJ}$
D) $7.9 \mathrm {~nJ}$