A bank is testing a new method for getting delinquent customers to pay their past-due credit card bills. The standard way was to send a letter (costing about $0.45) asking the customer to pay. That worked 30% of the time. The bank wants to test a new method that involves sending a DVD to customers encouraging them to contact the bank and setup a payment plan. Developing and sending the video costs about $10 per customer.

Question

A bank is testing a new method for getting delinquent customers to pay their past-due credit card bills.  The standard way was to send a letter (costing about $0.45) asking the customer to pay.  That worked 30% of the time.  The bank wants to test a new method that involves sending a DVD to customers encouraging them to contact the bank and setup a payment plan.  Developing and sending the video costs about $10 per customer.

20.
What is the parameter of interest?

A) past-due credit card bills
B) proportion of customers who are delinquent in paying their credit card bill
C) proportion of customers who pay their credit card bill on time
D) proportion of delinquent customers who pay their credit card bill

Would this be B?  I thought C, but got the answer wrong.