题目

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。How the earth exactly began is a mystery, because it【1】 (happen) so long ago. 【2】 is widely accepted is that the universe began with a “Big Bang”. For several billion years after the “Big Bang”, the earth was still just【3】 cloud of dust. Later, the earth exploded loudly with fire and rock, and they were in time to produce gases. As the earth cooled down, water, making it possible for life 【4】 (exist), appeared on its surface .Many millions of years later, more and 我国第一颗绕月探测卫星--“ ”于2007年10月24日晚在西昌卫星发射中心发射成功并进入预定地球轨道．