Here's a problem to solve: If a book sitting on a flat table starts to move horizontally, it is most likely moving because

Among the 4 following options, which is the correct answer?

- A: a balanced force is being applied.
 
- B: a frictional force is being exerted.
 
- C: an unbalanced force is being applied.
 
- D: a gravitational force is being exerted.
 