A person holds a book at rest a few feet above a table. The person then lowers the book at a slow constant speed and places it on the table. Which of the following accurately describes the change in the mechanical energy of the earth-book system?

A. the total mechanical energy is unchanged, because there is no change in the book's kinetic energy as it is lowered to the table.

B. the total mechanical energy is unchanged, because no work is done on the earth-book system while the book is lowered.

C. the total mechanical energy decreases, because the person does positive work on the book by exerting a force that opposes the gravitational force.

D. The total mechanical energy decreases, because the person does negative work on the book by exerting a force on the book in the direction opposite to its displacement.