Answer: 1) Industrial activity had increased, which involved burning fuels like coal. 2) The global human population is continuously increasing, straining resources. 3) There was an increasing number of fossil-fuel burning vehicles on the road.
Explanation: 1) is the burning on fuels like coal emit CO2 in the air contributing to the high levels of CO2. 2) The straining of resources is vague but, most likely some of those resources emit CO2 in the air. 3) the burning of fossil-fuels in vehicles on the road defiantly add to the emission of CO2 in the air. Double check if you like but, this is my answer