A、To give an example of a discovery that was unexpected. B、To illustrate the consequences of intense volcanic activity. C、To que

Narrator: Listen to part of a lecture in an astronomy class.
    Professor: Many people have been fascinated about Venus for centuries because of its thick cloud cover, this so-called planet of mystery and all of that. Well, what’s under those clouds? What’s the surface of the planet like? Some questions about the surface are still unresolved but, but we have learned a lot about it in the past several years.
    On Venus, however, volcanoes are not clustered in discrete zones like they are on Earth. Instead , they are more or less randomly scattered over Venus’s surface. Well, that’s significant. Venus has mostly shield volcanoes, and they are randomly scattered, that indicates that Venus does not have moving tectonic plates, and that’s a big difference compared to Earth. Here on Earth, moving tectonic plates are a major geological element, just crucial for the whole surface dynamic, right? So why doesn’t Venus have them? Well, there are a few theories. One of them is that this has to do with the fact that Venus has no surface water that’s needed to kind of lubricate the movement of the plates, you know, like oceans on Earth. Yeah, I forgot to spell that out. Uh, Venus has no surface water.
    Student: Wait a second. Did you say we have shield volcanoes on Earth? Can you give an example?
    Professor: Sure. The volcanoes in the Hawaii islands, in the Pacific Ocean are shield volcanoes. They are formed over a hot spot of magma. So while on Earth we have several types of volcanoes, on Venus there’s mostly, the one type. Uh, Eric?
    Student: Are the volcanoes on Venus still active?
    Professor: Well, that’s an interesting question. There is still some discussion on that point. But here’s what we do now. First, the level of sulfur dioxide gas above Venus’s clouds shows large and very frequent fluctuations. It is quite possible that these fluctuations, the huge increase and decrease of sulfur dioxide, happening again and again. It’s quite possible that this is due to volcanic eruptions, because volcanic eruptions often emit gases. If that’s the case, volcanism could very well be the root cause of Venus’s thick cloud cover. And also we have observed bursts of radio energy from the planet’s surface. These bursts are similar to what we see when volcanoes erupt on Earth. So this two suggests ongoing volcanic activity. But although this is intriguing evidence, no one’s actually observed a Venus volcano erupting yet, so we can’t be positive.
23. Why does the professor mention the fact that Venus has no surface water?
24. What is a possible origin of Venus’ clouds?
25. What is the professor’s opinion about the theory that Venus’ volcanoes are active?