If the Marginal Cost of Keeping a Dentist's Office Open

Multiple Choice

If the marginal cost of keeping a dentist's office open one additional hour per day is $300, then the dentist should keep the office open for one extra hour:

A) only if the marginal benefit she receives is greater than $300 plus an acceptable profit margin.
B) as long as the marginal benefit she receives is just equal to or greater than $300.
C) as long as the marginal cost does not rise.
D) until the marginal benefit she receives reaches zero.