A study of class attendance and grades among first-year students at a college showed that, in general, students that attended a higher percent of their classes earned higher grades. Class attendance explained 18% of the variation in grade index among the students. What is the numerical value of the correlationrbetween percent of classes attended and grade index?

r = - 0.424

r = 0.424

r = 1.80

r = 0.032