1. Why is outside air mixed with return air?

Explanation:

outside air must be mixed with return air at some point in time because here is an example a warm current of air comes in contact with a cold current of air  the warm current has to cool down so it could mix with the cold current or if they don´t mix they would continue to bump into each other and probably cause a tornado. probably.

answer:

person b

explanation:

B) Between two private individuals