Lily used to make customized mugs for her customers. She bought regular mugs, painted designs on them, and then resold them. Recently, she
bought a large stock of regular mugs because she could get them at a discount. Keeping her selling price the same, she was able to increase her profits by selling the additional customized mugs. Which factor explains Lily's increased profits with increased production?

Lily's increased profits can be explained by the concept of (blank)

there are no choices!