A、Their recent situations. B、The beautiful views in fall. C、Various colors of tree leaves. D、Color changes of tree leaves. D主旨大意

M: A fine day, isn’t it?
W: Yeah. How is everything going, Mike? Are you still busy with your program?
M: Yes, I’m still quite busy, and I have to finish the program before the deadline, which is before the end of this month. (19)So after a whole day’s hard work and all the noise in the city, a quiet walk in the woods can be restful.
W: (19)You’re right here. I feel the same.
M: (20)Look, it’s so pretty when the leaves are changing color. I’m glad we decided to come here.
W: Changes in nature always make things so beautiful. In the spring, the green grass and leaves make everything seem new. In the autumn when the leaves turn red, yellow and brown, they cover the ground like a blanket. It’s so amazing!
M: (21)Actually, tree leaves turn color because days in the autumn are shorter than in the summer, and the amount of daylight hours decreases. I’m sure this doesn’t sound very romantic, but it’s scientifically accurate.
W: Scientifically accurate or not, I just enjoy looking at the trees that can have a dozen different shades of color in the sunlight.
M: Exactly. I can’t agree with you more. What about coming here on a weekly basis from today on?
W: Good idea. It’s a deal.
19. What does the woman think of walking in the woods?
20. What are the two speakers mainly talking about?
21. Why do tree leaves change color in the autumn?