Which Two of the Following Are the Key Elements in Determining

Which two of the following are the key elements in determining whether or not a switch from a no-credit policy to a credit policy is advisable?
I.variable cost per unit
II.cash discount percentage
III.credit price
IV.default rate

A) I and III only
B) II and IV only
C) II and III only
D) I and IV only
E) III and IV only