Mike's Research Has Shown a Positive Correlation Between the Number

Mike's research has shown a positive correlation between the number of books in families' homes and the grade-point averages of the children in those families. Mike's results indicate

A) that the presence of many books in a home causes children to be more studious.
B) nothing, because the study did not take library borrowing into account.
C) parents who buy many books encourage their children to study.
D) the children of families with a large number of books at home tend to do well in school.