In the Experiments by Arnold,et Al

In the experiments by Arnold,et al.,why were some leaves on a cacao plant treated with endophytes and others not treated?

A) as a control for variation between plants
B) to reduce the amount of endophyte spores that were needed
C) the endophytes were able to spread from one leaf to another
D) to keep the Phytophthora from spreading to other plants
E) to reduce the number of plants that were needed