Fred produces pears in an Orchard in Washington. He hires labor, uses a tractor and owns the land with the trees. He could rent his tractor to other farmers for $50 per day. He could lease his land to other farmers or sell it, but it would take over a year to close such a deal. Fred's short-run variable costs include:

A. the cost of labor, the opportunity costs of renting the tractor and the opportunity cost of the land.

B.the cost of labor only.

C. the rental income he collects when he rents out the tractor.

D. the cost of labor and the opportunity costs of renting the tractor.