For Elliot's Dog-Walking Service, the Only Variable Input Is Labor

Multiple Choice

For Elliot's dog-walking service, the only variable input is labor. Elliot's labor costs are $300 a day and his service walks 30 dogs per day. To walk 31 dogs per day, his labor costs increase to $305 a day. The marginal cost of walking that 31st dog is

A) $5.
B) $9.83.
C) $19.52.
D) indeterminate from the information given.