Three cubes with sides in the ratio of 3 : 4 : 5 are melted to form a single cube whose diagonal is $$18\sqrt{3}$$ cm. The sides of the three cubes are:

A

21 cm, 28 cm and 35 cm

B

9 cm, 12 cm and 15 cm

C

18 cm, 24 cm and 30 cm

D

12 cm, 16 cm and 20 cm