Ascientist wants to determine which fertilizer is more effective—fertilizer x or fertilizer y. the best way for her to proceed would be to design an experiment with

a. two groups of plants—a group fertilized by y and a control group with no fertilizer.

b. two groups of plants—a group fertilized by x and a control group with no fertilizer.

c. three groups of plants—a group fertilized by x, a group fertilized by both x and y, and a control group with no fertilizer.

d. three groups of plants—a group fertilized by x, a group fertilized by y, and a control group with no fertilizer.