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.