At Elliott's dog-walking service, the only input is labor. Elliott's labor costs are $300 per day and his service walks 30 dogs per day. To walk 31 dogs per day, his labor costs would increase to $305 per day. The marginal cost of walking the thirty-first dog is_________ per day. 

a. $5

b. $9.83 

c. $19.52

d. $305 

e. $300