The interior walls of the small intestineare tightly wrinkled into projections called circular folds that greatly increase their surface area. Microscopic examination of the mucosa reveals that the mucosal cells are organized into finger-like projections known as villi, which further increase the surface area
The Answer is Heat Energy
I dont know 100% sure but C sounds the best. Humans breathe in oxygen (that plants make) and generate carbon dioxide and then releases it for plants to use.