A Researcher Is Interested in the Effects of a Vegetarian

A researcher is interested in the effects of a vegetarian diet on memory. One group eats a strict vegetarian diet for three months, a second group eats a strict vegan diet for three months, while a third group eats anything they want for three months. After three
Months, all participants are given a memory test. Results indicated that memory was is
Better for both the vegetarian and vegan groups. In this study, the independent variable is

A) the three month time frame.
B) impossible to tell in this situation.
C) the type of diet.
D) scores on the memory test.
E) the vegetarian diet but not the vegan diet.