Matilda Is Interested in Number of Books Read by Each

Multiple Choice

Matilda is interested in number of books read by each of the third grade children in her school district during the academic year. She believes that there may be differences in the number of books read by classroom and by school and she thinks there is a positive relationship between number of books read and reading achievement as measured on the standardized assessment given at the end of the year.
-In Matilda's study the variable 'school' represents which of the following type of variable?

A) Independent variable
B) Neutral variable
C) Manipulated variable
D) Dependent variable