阅读下面材料，在空白处填入适当的内容（1个单词）或括号内单词的正确形式。

    No one knows ____(exact) how the earth began, as it happened so long ago. However, according to a widely ____(accept) theory, the universe began ____a “Big Bang” that threw matter in all ____(direction). For several billion years after the “Big Bang”, the earth was still just a cloud of dust.____ it was to become was ____(certain) until between 4.5 and 3.8 billion years ago when the dust settled into a solid globe. The earth became so ____(violence) that it was not clear ____the shape would last or not. It exploded loudly with fire and rock. They were in time ____(produce) carbon dioxide, nitrogen, water vapour and other gases, which were to make the earth's atmosphere. What is even more important is that as the earth cooled down, water began to appear on ____ surface.