Asolid metallic cube of side 20 cm is melted and recast into a cuboid of length 40 cm and breadth 40 cm, Whatis the length (in cm)of the body diagonal of the cuboid?

A

$$15\sqrt{43}$$

B

$$129\sqrt{5}$$

C

$$43\sqrt{15}$$

D

$$5\sqrt{129}$$