Read the passage below and choose the best answer that expresses the main idea of the passage: people concerned with the environment and global warming usually try to reduce their carbon output by increasing their home's energy efficiency and driving less. some start by calculating their carbon footprint to set a benchmark -- like a weigh-in before a diet. a carbon footprint is simply a figure -- usually a monthly or annual total of co2 output measured in tons. a. everyone should reduce their carbon output. c. people concerned with global warming should know their carbon footprint so they will know how they can reduce their carbon output. b. you should always calculate your carbon footprint before going on a diet. d. everyone puts out tons of co2 into the atmosphere every day.