Enny was reviewing the water bill for her doggy day spa and determined that her highest bill, $1,700, occurred in July when she washed 800 dogs and her lowest bill, $900, occurred in November when she washed 400 dogs. What was the variable cost per dog associated with Jenny's water bill?
a) $0.67
b) $1.00
c) $0.50
d) $2.00