A Group of Researchers Is Investigating the Effects of Gingko

A group of researchers is investigating the effects of gingko biloba on memory. During the first part of the study the animals learn to run a maze while they are not receiving the supplement; in the second part of the study the animals learn to run a different maze while they are receiving the supplement. In each case the researchers count how many trials it takes before the animals can run the maze pattern without making any errors. In this study, the independent variable is

A) the type of animal that the researchers select for the study.
B) the presence or absence of the food supplement in the animal's diet.
C) the number of trials it takes to run the maze without making any errors.
D) the age of the animals in the study.