Final answer:
The opportunity cost for Mrs. Clark watching a movie instead of grading tests is the test grading she did not complete. Opportunity cost represents the most desirable alternative given up when making a choice, similar to Selena's decision in viewing a terrible movie.
Step-by-step explanation:
If Mrs. Clark decides to watch a movie for two hours but could have finished test grading in those two hours, the opportunity cost of this decision to watch a movie is the grading of tests that she did not complete.
Opportunity cost is a key concept in economics that refers to the most desirable alternative foregone as a result of choosing one option over another. In Mrs. Clark's case, by choosing to watch a movie, she foregoes the valuable time she could have spent grading tests. This is similar to the concept of sunk costs as illustrated by Selena's case, where Selena must decide whether to watch the rest of a terrible movie or do something else with her time. The money spent on the ticket is a sunk cost, while the time she could spend doing something more enjoyable than watching the movie is the opportunity cost.