单选题   A group of scientists rowing toward the center of a lake saw something shocking. They turned back as fast as they could. What had they seen? The lake was boiling!
    The group was investigating a crater lake in the mountains of St. Vincent. A crater lake is the mouth of a volcano that has been dormant for some time and has filled with water.
    This particular crater was the tip of a volcano called Soufriere, which erupted last in 1902. Since that time, it had not shown any signs of action. But in the fall of 1971, mountain climbers who had hiked near the lake returned to the lowlands with strange stories. They said the water had turned yellow and was giving off a smell like burnt eggs. A seething fog was rising from the lake's surface.
    Local scientists rushed to Soufriere to see if this might be the beginning of a new volcanic explosion. They found a huge black mass in the middle of the water. It was a great blob 1,000 feet long and 300 feet wide. Lava had pushed up through the bottom of the lake and formed a new island.
    The investigators wanted to make sure that the volcano was safe, and that the lava would not overflow into the surrounding countryside. But they could never reach the island to study it, because the lava was so hot that the water around it bubbled and boiled.
单选题     This passage is about ______.
 
【正确答案】 C
【答案解析】这篇文章讲的是火山熔岩堆积的新岛。选项C是正确的。选项A和B是文中提到的,但并不是文章的主题。
单选题     A crater lake is at the tip of a ______.
 
【正确答案】 D
【答案解析】第二段。解释crater lake是火山的口,里面装满了水。选项D是正确的。
单选题     We can conclude that ______.
 
【正确答案】 B
【答案解析】第三段。Soufriere火山自1902年以后就再也没有爆发过,然而1971年,几个登山者的发现,以及后来科学家的调查,都说明它是活火山。选项B是正确的。
单选题     The writer explains the boiling water by using ______.
 
【正确答案】 C
【答案解析】第三、四段。作者用事实说明水的沸腾。如,水的颜色和气味等。选项C是正确的。