The edge of three cubes of metal is 3 dm, 4 dm and 5 dm. They are melted and formed into a single cube. Find the edge of the new cube?  Choose the correct answer.
A) 2
B) 8
C) 7
D) 6
E) 9

Rationale: 33 + 43 + 53 = a 3 = > a = 6 answer : d
Answer: D