What is an example of active transport?

Active transport refers to a substance moving from areas in which it has low concentration to an area with high concentration, one example i guess is endocytosis, "the taking in of matter by a living cell by invagination of its membrane to form a vacuole"

A common example of active transport, or moving a substance against its gradient, is the maintaining of a balance of sodium and potassium inside and outside a cell. Obviously too much sodium inside the cell would cause cell death. So, the cell must actively regulate how much sodium is allowed in through the membrane.

Is there any answer choices?