Narrator Listen to a part of a discussion in a biology class. What is the talk mainly about?

[Professor] Now, let’s have a look at the bees. Until recently, we knew almost nothing about how important bees are in maintaining natural diversity. Now we know more about them. We know, for example, that honeybees are the dominant pollinators because they play a role in pollinating four out of five food crops in North America. We also know that honeybees, along with the other insects, bats, and birds that transfer pollen between flowers, all together they contribute more than ten billion dollars a year to fruit and see production on North American farms.
[Student A] Wow, It’s amazing. Pollination is so important for farming. [Professor] Yes, it is very important. Pollination is one of nature’s services to farmers. Just imagine about this: if we don’t have the pollinators, we would not have enough food to eat. We biologists never imagined we’d see the day when wild plants or crops suffered from pollinator scarcity. But, unfortunately, that day has come. In factor, farmers in Mexico and the states are suffering the worst pollinator crisis in history. So... what happened? Any idea?
[Student A] Is it... um... because of natural enemies? I read something about a kind of parasite that’s killed lots of bees.
[Professor] It’s true. An outbreak of parasite mites has caused a steep decline in North American populations of honeybees. But parasites aren’t the only factor.
[Student B] What about the pesticides used on farms? All those chemicals must have an effect.
[Professor] Most definitely, yes. Pesticides are a major factor. Both wild and domesticated bees are in serious trouble because of pesticides. In California, farm chemicals are killing around ten percent of all the honeybee colonies. Agriculture in general is part of the problem. Think about this for a minute, the North American continent is a vast collection of "nectar corridors" made up of flowering plants. Is there anybody who knows anything about these corridors?
[Student A] Yes. These corridors stretch for thousands of miles, from Mexico to as far north as Alaska.
[Professor] That’s right. And every year, there’s an array of migratory pollinators flying north and south with the seasons, following the flowers. The migratory corridors, the flyways... are like... uh... something like a path of stepping stones for the pollinators, with each "stone" being a collection of flowering plants. But our system of large scale agriculture has interfered. During the past fifty years millions of acres of desert in western Mexico and the southwestern United States have been turned into chemically intensive farms, planted with exotic grasses, creating huge stretches of flyway that are devoid of nectar producing plants for migratory pollinators. What we have now are huge gaps between the stepping stones... patches of plants here and there. A couple of migratory pollinators are worth noting. One is the lesser long nosed bat, and another is the most famous pollinator... what is our most famous pollinator? Or I should say our beautiful pollinator.
[Student B] Oh, I know. It’s the monarch butterfly!
[Professor] The monarch butterfly...yes. Millions of monarch butterflies all over the U.S. and southern Canada fly south every year in late summer. The monarch is the only butterfly that returns to a specific site year after year. Unfortunately the herbicides used on the milkweed in the Great Plains are taking a toll on monarchs, and fewer of them are reaching their winter grounds in Mexico. Another important pollinator is the long nosed bat. These amazing animals feed on cactus flowers. What they do is, they lap up the nectar at the bottom of the flower, and then when the bat flies off to another cactus, the pollen stuck to its head is transferred to that plant’s flower. But the long nosed bat is having a tough time, too. Some desert ranchers mistake them for vampire bats, and they’ve tried to poison them, or dynamite the caves where they roost.
29. What is the talk mainly about?
30. Based on the professor’s talk, what factors have contributed to the decline of pollinator populations? Click on two answers.
31. According to the professor, which is the most famous pollinator?
32. Based on the discussion, on the "steeping stones", what does the "stone" refer to?
Narrator  Listen again to a part of the conversation. Then answer the question.
[Professor] But our system of large scale agriculture has interfered. During the past fifty years millions of acres of desert in western Mexico and the southwestern United States have been turned into chemically intensive farms, planted with exotic grasses, creating huge stretches of flyway that are devoid of nectar producing plants for migratory pollinators. What we have now are huge gaps between the stepping stones...patches of plants here and there.
33. Why does the professor say thisΩ?
Narrator  Listen again to a part of the conversation. Then answer the question.
[Professor] Millions of monarch butterfly all over the U.S. and southern Canada fly south every year in late summer, The monarch is the only butterfly that returns to a specific site year after year. Unfortunately the herbicides used on the milkweed in the Great Plains are taking a toll on monarchs, and fewer of them are reaching their winter grounds in Mexico.
34. What can be inferred about monarch butterflies?