Question $18(1$ point$)$

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

A$)$ $$9\mathrm{.}83.$

B$)$ indeterminate from the information given.

C$)$ $$19\mathrm{.}52.$

D$)$ $$5.$