In the text below some words are missing. Drag words from the box below to the appropriate place in the text. To undo an answer choice, drag the word back to the box below the text.
下面的文本缺了一些词。请把词框里的词拖到文中合适的位置；要撤销选择，把词拖回文本下方的词框即可。
Volcanoes blast more than 100 million tons of carbon dioxide into the atmosphere every year but the gas is usually . When a volcano erupts, carbon dioxide spreads out into the atmosphere and isn't in one . But sometimes the gas gets trapped under enormous pressure. If it escapes to the surface in a dense , it can push out oxygen-rich air and become deadly.
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