Tammy is building a framework for a doorway for a house under construction. The door is in the shape of a rectangle. The height of the door is represented by the expression 5x-6. To allow for the maximum amount of space in the entryway, the area is represented by the expression 30x-36 square feet. What is the base of the door in feet?
A. 4 feet
B. 2 feet
C. 8 feet
D. 6 feet