文章大意：这是一篇说明文。众所周知，美国黄石国家公园是天然火山，但是火山学家Bob Christian-sen一直找不到火山口，很困惑。后来宇航局送来的几张照片令他恍然大悟，原来整个黄石公园就是一个巨大的火山口。 只是这座火山并不是像我们大部分人想象的那种圆锥体形状的高耸的样子，而是一个巨大的洞，这个洞太大了，以至于从地面上的任何地方都看不见。
1 . In the 1960s, while studying the volcanic history of Yellowstone National Park, Bob Christian-sen became puzzled about something that, strangely, had not troubled anyone before he couldn’t find the park’s volcano. It had been known for a long time that Yellowstone was volcanic in nature—that is what accounted for all its hot springs and other steamy features. But Christian-sen couldn’t find the Yellowstone volcano anywhere.
Most of us, when we talk about volcanoes, think of the classic cone(圆锥体)shapes of a Fuji or Kilimanjaro, which were created when erupting magma(岩浆)piled up. These can form remarkably quickly. One day in 1943, a Mexican farmer was surprised to see smoke rising from a small part of his land. In one week he was the confused owner of a cone five hundred feet high. Within two years it had topped out at almost fourteen hundred feet and was more than half a mile-across. Altogether there are some ten thousand of these volcanoes on Earth, all but a few hundred of them extinct. There is, however, a second less known type of volcano that doesn’t involve mountain building. These are volcanoes so explosive that they burst open in a single big crack, leaving behind a vast hole, the caldera(火山口).Yellowstone obviously was of this second type, but Christian-sen couldn’t find the caldera anywhere.
Just at this time NASA decided to test some new high-altitude cameras by taking photographs of Yellowstone. A thoughtful official passed on some of the copies to the park authorities on the assumption that they might make a nice blow-up(放大的照片)for one of the visitors’ centers. As soon as Christian-sen saw the photos, he realized why he had failed to spot the caldera: almost the whole park—2.2 million acres -was a caldera. The explosion had left a hole more than forty miles across-much too huge to be seen from anywhere at ground level. At some time in the past Yellowstone must have blown up with violence far beyond the scale of anything known to humans.
1. What does the underlined word “its” in paragraph 1 refer to?
| A．The ground. | B．The nature. |
| C．The volcano. | D．The park. |
2. Why was there smoke rising from the Mexican farmer’s land2
| A．A single big crack was forming there. | B．Someone was burning stalks there. |
| C．A volcano was erupting underneath. | D．There were some hot springs there. |
3. How did Bob Christian-sen discover the park’s volcano?
| A．By looking at the photographs. | B．By analyzing its steamy features. |
| C．By researching historical documents. | D．By using some advanced equipment. |
4. What can we infer about Yellowstone?
| A．Its level is much higher than the surrounding areas. |
| B．The volcano in it involved mountain building. |
| C．It is a large hole left by the explosion. |
| D．Its shape is much like a cone. |
