A、By observing the ordinary ones that have already erupted. B、By studying the larger part of a volcano hidden underground. C、By

M: Good morning, class.
W: Good morning, sir.
M: Today we’ll discuss volcanoes. Can you tell me the most dangerous thing about a volcano?
W: I think the most dangerous hazard of a volcano comes from the lava that flows out of it.
M: Good guess. [19]Although your answer makes sense, the biggest killer from a volcano comes from the ash, not the lava. In fact, ash causes serious problems and can make humans and animals sick; it can even kill them. Ash also reduces sunlight and can start heavy rainfalls that lead to mudslides. Did you know that it only takes 30 centimeters of dry ash to collapse a roof of a building? Ash also contaminates water supplies, and kills crops. As you can see, ash causes the most destruction.
W: Professor, are there different sizes of volcanoes?
M: Absolutely. There are ordinary volcanoes that can kill thousands of people and often destroy whole cities, and there are super volcanoes which are highly explosive and can kill billions of people and even destroy an entire continent.
W: Are there any super volcanoes in the world today?
M: Actually, yes. There are 40 super volcanoes. One is in Yellowstone National Park in the United States and two others in Britain. Super volcanoes are quite rare with the last eruption occurring about 74,000 years ago in Indonesia.
W: How do super volcanoes differ from ordinary volcanoes and are they easy to find?
M: By and large, most people would not be able to recognize a super volcano. Like an iceberg, the larger part of a volcano is hidden.
W: Professor, if the volcano hasn’t erupted then how do scientists know what might happen?
M: Well, [21]the scientists are relying on past information from the volcano that erupted in Indonesia and they also forecast their predictions using computer models.
19. What is the most dangerous thing about a volcano?
20. Where did the last eruption of super volcano occur?
21. How can scientists predict the future of volcanoes?