Question: Which of the following statements best explains why the lower mantle of Earth is much more rigid and dense than the upper mantle?

  Options: 
 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. 
Answer: C