28. Which of the following have researchers long been doubtful about?
A. Beavers can help control wildfires
B. Beavers can build dams and form ponds
C. Beavers can benefit the ecology of the earth
D. Beavers can offer every vital service humans need
29. How did the researchers conduct the research?
A. They analyzed the number of beavers
B. They observed satellite images
C. They measured the size of the plants
D. They recorded the changes of ecosystems