Dr. Zelke surveys 50 university students to discover the relationship between textbook price and ratings of readability. Dr. Zelke finds that for these two variables the correlation coefficient is -0.70 . This indicates that

a. more expensive books tend to receive higher readability ratings than less expensive books.
b. there is no relationship between textbook price and ratings of readability.
c. increasing a textbook's price will cause a decrease in its readability rating.
d. more expensive books tend to receive lower readability ratings than less expensive books.

D