In the Section "Investigating Life: the Battle for Position in Cacao

In the section "Investigating Life: The Battle for Position in Cacao Tree Leaves",why were some leaves on a cacao plant treated with endophytes and others not?

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