Joyce Huang asked in Science

My answer was 4 but the correct answer turned out to be 3. Please give me a through explanation, it will be appreciated.

Thank You

Nikhil answered this

Dear student 

(a) could also have been the correct answer because When you plant trees (reforestation),you are directly cleaning the air. As a tree matures, it can consume carbon dioxide per year as it turns that CO2 into parts of itself. But we can consider here that since reforestation takes many years ie. minimum 65 years this could be the reason of it not included.
So, (3) is the correct answer.