The equation CO2 = P × S × E × C shows how the total amount of carbon dioxide emissions on Earth (CO2) is calculated. The number of people (P) is multiplied by the services, like electricity, that the people use (S). This is multiplied by the energy needed for each service (E) and then by the amount of carbon dioxide released for each unit of energy (C). In order for the total emissions to be 0, why must one of the variables on the right side of the equation be 0? Realistically, why is C the only variable that can become 0?