Read the passage below and choose the best answer for the main idea of the passage: footprints offer clues about where we came from and where we're headed. their impressions tell us something about the animals that leave them. but while actual footprints offer details on size, weight and speed, carbon footprints measure how much carbon dioxide (co2) we produce just by going about our daily lives. a drive to work, a flip of a light switch and a flight out of town all rely on the combustion of fossil fuels like oil, coal and gas. when fossil fuels burn, they emit greenhouse gases like co2 that contribute to global warming. ninety-eight percent of atmospheric co2 comes from the combustion of fossil fuels. a. footprints come in all shapes and sizes. c. global warming comes from the emission of co2 into the atmosphere. b. we study footprints to learn about the animals who leave them. d. no one should be allowed to drive because it causes global warming.