A gardener starts a compost pile. He adds dead leaves, lawn clippings, and other organic matter to the pile. After a few weeks, the gardener notices that materials in the pile have turned brown and soft. The compost pile also feels warm both during the day and at night. Which of the following statements is the best explanation for this?
A. Wind is causing the materials in the pile to rub together, and this generates heat from friction.
B. Wild animals are using the compost pile as a nest, and their body heat is warming the pile.
C. Soil from the ground has moved up through the compost pile, which makes the pile denser and warmer.
D. Decomposers are breaking down materials in the compost pile, which causes heat energy to be released.