A、Mid-July. B、July. C、Late August. D、At least in September. D从“These moths continue to pollinate(授粉)the flowers until well into

Today I’d like to discuss the first non-instance of strange evolutionary strategy. This phenomenon occurs in a flowering plant called the scarlet jilia. The scarlet jilia depends on hummingbirds and moths for pollination. And the blossoms seem to change their color for each of these pollinators. On fern mountains near Flagstaff, Arizona where these flowers grow, hummingbirds arrive in mid-July to drink nectar from the scarlet jilia. In July, the flowers are bright red, which is attractive to the hummingbirds. By late August, however, the hummingbirds leave the area for a warmer climate. As the weather cools, the color of the plant blossoms changes to white, a color that attracts a particular moth species, the night-flying hawk moths. Apparently, this kind of moths can see these white blossoms easily at night. These moths continue to pollinate the flowers until well into September. The scarlet jilia demonstrates at an unusual way in which the plant lures pollinators to continue its life cycle.
Questions 16-20 are based on the passage you have just heard.
16. What’s the main topic of the talk?
17. Why is the scarlet jilia plant considered to be unusual?
18. Why do the hummingbirds leave fern mountains?
19. When is the scarlet jilia plant pollinated?
20. According to the speaker, in what way are white blossoms attractive to moths?