In the Sonoran Desert of the Southwestern United States, Scientists

In the Sonoran desert of the southwestern United States, scientists observed that population clusters of a small cactus were pronounced around nests of desert woodrats. Given this spatial relationship, they began to think the woodrat was connected to the seeding of these cactus plants. 
After years of counting and observing woodrat nests and nearby cactus plants, scientists collected woodrat droppings for laboratory analysis to determine whether they contained cactus seeds. They also excluded woodrats from several other areas without cactus plants, then collected woodrat droppings and added those to half of these areas to examine whether the droppings were associated with the growth of new population clusters of the cactus.
Scientists then programmed the data they had collected into a computer. A computer program enabled them to predict that increases in the woodrat population would result in corresponding increases in the cactus population, and likewise, a decrease in the number of woodrats would signal an impending decrease in cactus.
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Which phase of the study could be described as modeling?

A) when the scientists gathered woodrat droppings for lab analysis
B) when the scientists determined from computer programs how woodrat nest numbers would impact cactus populations
C) when the scientists first observed the proximity of woodrat nests and cactus plants
D) when the scientists transported woodrat droppings to other areas
E) when the scientists first wondered whether or not woodrats were connected to the seeding of the cactus plants