Imagine that you and a person very similar to you, live in separate apartments and you each use about $50$ units of hot water per month.

A$.$ If you decide to share an apartment, will your combined hot water use be greater than $100$ units, less than $100$ units or very close to $100$ units? Explain your answer from an economic perspective.

B$.$ How does your answer change if you marry this person?

C$.$ What does this have to do with the environment?

Hint: Answers involving added efficiencies in living together or even in showering together are interesting, but not the point of this question so please don't tell me about them. There's something different and more fundamental going on here. If you like, consider the extreme example of sharing accommodations with $1,000$ people as I did in a dormitory.