Answer:
c. the amount of fossil fuels being burned increased
Step-by-step explanation:
Carbon dioxide gas concentration increased over the time due to burning of fossil fuels increased. Fossil fuels are organic compounds which releases carbondioxide gas as a result of burning of fossil fuels in the engines of vehicles and industries. With the passage of time, number of vehicles increases in the world which needs more fossil fuels and due to burning of that fossil fuels, more carbondioxide gas is released.