Here's a problem to solve: Which of the following statements best explains why the lower mantle of Earth is much more rigid and dense than the upper mantle?

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

- A: The lower mantle is older than the upper mantle.
 
- B: The lower mantle is cooler than the upper mantle.
 
- C: The lower mantle is under more pressure than the upper mantle.
 
- D: The lower mantle is farther from the core than the upper mantle.
 