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