Answer:
$1 per square foot per month
Explanation:
the marginal cost of renting an apartment with 500 additional square feet = $1,700 - $1,200 = $500 or $1 per square foot per month.
The marginal cost of anything is the cost of purchasing (or leasing or producing) one extra unit of output or product. This is why the answer must be expressed in the cost of an extra unit, not 500 extra units.