Here's a problem to solve: Which type of force requires contact between two objects for one to push or pull the other?

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

- A: frictional forces slowing down a rolling soccer ball
 
- B: the magnetic force pulling paper clips to a powerful electromagnet
 
- C: the magnetic force pushing two magnets apart
 
- D: the force of gravity acting on raindrops that fall to Earth
 