单项选择题

Volcanoes
There are thousands of volcanoes (火山) all over the world. What makes volcanoes What happens
The inside of the earth is very hot. Because it is very, 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 appears. The hot melted rock, which we call "lava" (熔岩), pushes out 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. Perhaps the volcano will then be quiet. Perhaps it will start again hundreds of years later.
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. A great cloud of smoke shot up into the sky with great burning rocks. Hot lava poured clown its sides. About 3,000 people were killed.
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. You can see that a volcano can stay alive for many years. There was also serious damage in 1914 hut there has not been any since that year. According to the passage, a volcano sends out ______.