Which activity involves using fossil fuels directly or inderectly

a: cooking hot dogs on a campfire

b: using crayons to color

c: walking to school

d: eating fresh fruit for snack

The large increase in atmospheric carbon dioxide in the last 50 years most likely comes from a. an increase in cellular respiration b. increased decomposition by bacteria c. an increase in the burning of fossil fuels d. an increase in photosynthesis