△ABC and △DEF are similar. The altitude from A to BC measures 4.5cm and the corresponding altitude from D to EF measures 6cm. If BC=7.5cm and AB=6cm, answer the following:
a. (i) Find the ratio of the sides of △DEF to those of △ABC.
b. (ii) Calculate the length of EF.
c. (iii) Determine the length of DE.