A Group of Researchers Investigates the Effects of a Vitamin

Multiple Choice

A group of researchers investigates the effects of a vitamin supplement on animal 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.What is the independent variable in this study

A)  the number of trials it takes to run the maze without making any errors 
B)  the trials in which the supplement is used 
C)  the presence or absence of the supplement in the animal's diet 
D)  the two different mazes used