Rather Than Go Out to Eat by Yourself, You Decide

Essay

Rather than go out to eat by yourself, you decide to stay at home and fix dinner for yourself and your two roommates.Your roommates applaud your decision.
a. Your first roommate tells you that your decision to eat at home has no opportunity cost because you already have all the dinner ingredients in your pantry.Is this roommate's comment correct?
b. When preparing dinner, you use some milk that has a pull-date of tomorrow, so that the milk would not be good tomorrow.Your second roommate notices the date on the carton, and comments that the cost of the milk you are using is a sunk cost.Is this roommate's comment correct?