The amount of concrete, c, needed to make a walkway around a circular swimming pool varies directly as the square of the diameter, d, of the pool. Which statement is true? A. If the diameter of the pool is doubled, the amount of concrete needed is multiplied by 2.

B. If the diameter of the pool is doubled, the amount of concrete needed is multiplied by 4.

C. If the diameter of the pool is halved, the amount of concrete needed is multiplied by 2.

D. If the diameter of the pool is halved, the amount of concrete needed is multiplied by 4.