第三部分：语法填空（共两节，满分20分，每小题1分）
根据文段内容或提示，填写正确的单词，每空只能填写一个单词。
No one knows exactly __31___ the earth began, as it happened so long ago. However, according to a __32___ (wide) accepted theory, the universe began ___33___ a “Big Bang” that threw matter in all directions…..What it was to become was __34___(certain) until between 4.5 and 3.8 billion years ago __35___ the dust settled into a solid globe. The earth became so violent __36___ it was not clear whether the shape would last or not…….People are__37__ (put) too much carbon dioxide into the atmosphere, which prevents heat from __38____(escape) from the earth into space. As a result of this, many scientists believe the earth may become too hot to live on. So __39____life will continue on the earth for millions of years to come will depend on whether this problem can be __40___(solve).