Questions 51 to 55 are based on the following passage. 　　There are thousands of volcanoes all over the world. What makes volcanoes What happens 　　The inside of the earth is very hot. Because it is very hot, the rock has melted like ice. It has become liquid, like water. It is always boiling, like water in a kettle. If you have seen a kettle boiling, you know that the steam and boiling water try to get out. The very hot melted rock inside the earth also tries to get out. Usually it cannot because the outside of the earth is too thick and strong. 　　But in some places the outside of the earth is thin and weak. Sometimes a crack （a small opening） appears. The hot melted rock , which we call “lava”, pushes up through the crack and bursts through . Steam and gas shoot up into the air and the hot melted lava pours out. Big pieces of rock may be thrown high into the air. 　　After a while the volcano becomes quiet again. The melted lava becomes hard. Later the same thing happens again and again. Each time more hot lava pours out on top of the cold lava and then becomes hard. In this way a kind of mountain is built up, with a hole down the middle. 　　Although there are thousands of volcanoes in the world, most of them are dead. Only about 500 sometimes start to throw out lava again. A famous volcano which is now dead is Mount Fujiyama in Japan. It is covered with snow on winter. 　　Vesuvius is the name of a very famous volcano in Italy. It first came to life many, many years ago. It was quiet for hundreds of years. Then in the year 79 it suddenly burst. This has happened again many times since that year. Sometimes no damage was caused or only little damage. But there was serious damage in the years 472, 1631, 1794, 1861, 1872 and 1906. 　　Statements: According to the passage, Vesuvius has caused serious damage 6 times.