Suppose you want to write a program that reads customer records and displays a summary of the number of customers who owe more than $1,000 each, in each of 12 sales regions. Customer fields include name, zip Code, balance Due, and region Number. At some point during record processing, you would add 1 to an array element whose subscript would be represented by _________.
a. name
b. zip Code
c. balance Due
d. region Number