Refer to the Figure

Multiple Choice

Refer to the figure.
Vredenburg studied the impact of introduction of rainbow and brook trout on a species of amphibian, the mountain yellow-legged frog (Rana muscosa) . After showing greatly decreased frog populations in lakes with trout, he set up an experiment using three categories of lakes: lakes from which he removed trout, lakes that had never had trout, and lakes that still contained trout. He compared the number of adult frogs per 10 m of shoreline in all groups, as shown in the figure. In Vredenburg's experimental design, why did he use two control groups?

A) To be doubly sure his results were correct
B) To ensure that he had enough data for statistical analysis
C) To control for unknown factors that might be present in the lakes
D) To control for the effects of two known natural conditions