An Ideal Parallel-Plate Capacitor Having Circular Plates of Diameter $D$ That Are a Distance

An ideal parallel-plate capacitor having circular plates of diameter $D$ that are a distance $d$ apart stores energy $U$ when it is connected across a fixed potential difference. If you want to triple the amount of energy stored in this capacitor by changing only the size of its plates, the diameter should be changed to

A) $\frac { D } { 3 }$
B) $3 D$ .
C) $D \sqrt { 3 }$
D) $\frac { \mathrm { D } } { \sqrt { 3 } }$
E) 9D.