At Elliot's shoe factory, when they produce 30 pairs of shoes their total cost is $500. Elliot's marginal cost to produce the 31st pair of shoes is $61. What is the average total cost of 31 pairs of shoes? Select an answer and submit. For keyboard navigation, use the up/down arrow keys to select an answer. a $16.13 b $16.67 c $18.10 d $61.00