What is an example of thinking at the margin?

The full question is:

What is an example of thinking at the margins

A figuring out what you will give up and what you will gain by hiring a new worker

B thinking about whether or not to go on a vacation to a warm beach

C drawing plans to build an extra room on an old home

D deciding whether to have a dog or a cat as a pet

The answer is A figuring out what you will give up and what you will gain by hiring a new worker.