第2题: [单项选择]If the radio waves had reached our planet one hundred years before, we would have______. A. sent an immediate answer B. sent an alarm against extraterrestrial attack C. sent a short reply D. sent no answer 参考答案:D 答案解析:[分析] 见第四段第三句话:我们自己的星球已经存在了50亿年,可能20亿年前出现生命,但只是在50年前,我们才向太空发射电波。因此,如果一百年前有电波到达地球,我们什么都不知道。