Variable costs are:
sunk costs.
the change in total cost associated with the production of an additional unit of output.
costs that change every day.
costs that change with the amount of output a firm produces.

the change in total cost associated with the production of an additional unit of output.

Explanation:

A variable cost is a cost that changes in relation to variations in an activity. In a business, the "activity" is frequently production volume.

a recipe serves 8 people, and calls for 5 lbs of apples and 2 cups of flour. you want to prepare it for 24 people. divide 24 by 8 to get 3, and then multiply your ingredients by 3.

so for 24 people, you'll need 15 lbs of apples, and 6 cups of flour.