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。
On average, Iceland a major volcanic event once every 5 years. Since the Middle Ages, a third of all the lava that has the earth's surface has erupted in Iceland. However, according to a recent geological hypothesis, this estimate does not include eruptions, which are much more extensive than those on the land surface.
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