请教一个简单的数学题

孩子有个数学题来问我，原题是：

Elaine makes a $4 purchase and gives the cashier a $20 bill.  If she has an equal probability of receiving any of the possible combinations of bills(out of $1, $5, or $10 bills)for change, what is the probability that she will receive a $5 bill among her change?

答案选择：

A. 1/8  B. 1/4  C. 3/8  D. 1/2  E. 5/8

我和他的答案都是 １／３，可不在答案选择中。请高手指点。多谢先！