Harvester ants often strip a bush of all of its leaves. Some people believe this helps the plant grow thicker, healthier stems. In an experiment, a student stripped off all the leaves from a set of plants. In a second set of identical plants, the student allowed ants to strip off the plants' leaves. In order to improve this experimental design, it is most important to add a set of plants :

a. with the same amount of water
b. of different species
c. with their leaves on
d. with different species on ants on them