Dr. Nguyen is studying how the quality of breakfast influences academic performance for elementary school children. Asample of 100 children is selected. Fifty children are provided with a nutritious breakfast when they arrive at school each morning. The other 50 children are given a doughnut each morning. After three months, Dr. Nguyen records the academic achievement level for each child and compares the two groups of scores. In this example, how many levels are there for the independent variable?
a. 1
b. 2
c. 50
d. 100