The Owner of a Bookstore in a College Town Notices

The owner of a bookstore in a college town notices that the demand for certain textbooks peaks during certain times of the year. She assumes that these peaks coincide with the beginning of semesters when professors recommend these books to the students. She decides to estimate the future sales of these textbooks by measuring the relationship between the sales of these books and the announcement of courses that recommend them. Which of the following methods of sales forecasting is she using?

A) Jury of executive opinion method
B) Sales force composite method
C) Time-series analysis
D) Correlation analysis