A study of 160 students, who took at least one math course, conducted at Summerisle Community College, revealed that 60% of students who have taken Introduction to Statistics graduate in four years, 45% of students who have taken College Algebra graduate in four years, and 75% of students who have taken Calculus graduate in four years. What variables are being studied?
A. How long it takes a student to graduate
B. Math classes a student takes
C. The number of students who take College Algebra
D. The number of students who graduate in four years