20. According to the article, how has observing the decline of big trees most affected the author?
O A He became active in reducing carbon emissions in
order to protect big trees.
B He expanded his previous research of American big
trees to become a global study.
C He expanded his research, intending to further explain
and publicize the causes of big trees' decline.
D He was relieved and encouraged by people's
enthusiasm to protect big trees and the environment.