A Calculus Book Weighing 20 N Rests on the Floor

A calculus book weighing 20 N rests on the floor of a classroom. The reaction to the force of the floor on the book is a force of

A) 9.8 m/s² exerted by the Earth on the book.
B) 0 N.
C) 2.04 kg exerted by the floor on the book.
D) 20 N exerted by the book on the Earth.