Answer:
The correct answer is "400 parts per million".
Step-by-step explanation:
According to Jessica Blunden's article "2013 State of the Climate: Carbon dioxide tops 400 ppm", over the past decade the average rate of CO2 concentration in the atmosphere has triplet. This caused that by 2012, a new threshold has reached and the World Resources Institute reported a CO2 concentration of 400 ppm. Unfortunately by 2013, the CO2 concentration increased, and was 2.8 ppm higher than in 2012.