The US Mint charges $25 for a limited edition coin, plus a $6 shipping charge. The
cost, c, of purchasing n coins can be found using the function c = 25n+6. There is a
limit of 5 coins per purchase. What is a reasonable domain for this function?
a. {5}
b. {1, 2, 3, 4, 5}
c. {25, 50, 75, 100, 125}
d. {31, 56, 81, 106, 131}