Algebra word question. time sensitive

A farmer wants to enclose a rectangular field along a river on three sides. If 4,000 feet of fencing is to be used, what dimensions will maximize the enclosed area? Select one: a. 1,000 ft (parallel to the river) by 1,000 ft b. 1,000 ft (parallel to the river) by 2,000 ft c. 2,000 ft (parallel to the river) by 1,000 ft d. 2,000 ft (parallel to the river) by 2,000 ft