Mike is designing a new brand of earphones. He needs to make the ear phones small enough to fit inside a person's ears. But if the earphones are too small, the speakers inside of them will not be able to generate a high-quality sound. Mike also knows that the earphones will be used by people as they exercise. So he needs them to be able to withstand sweat as well as hot and cold weather.

Which of the following is a constraint on Mike's design?

the desired sound quality
the size of an average person's ear
how durable the materials he uses are