翻译：史提芬·霍金认为地球不可能是唯一一个生命逐渐发展起来的星球.
此题同样考查定语从句的关系词的用法.
1.将先行词the planet代入定语从句中去,只能变成：
where life has developed gradually
= on which life has developed gradually
= life has developed gradually on which
= life has developed gradually on the planet
生命在这个星球上逐渐发展起来.
2.这就说明该定语从句缺少地点状语,所以用关系副词where充当.
其他ACD是关系代词,不能在句子充当状语,而只能充当主语/宾语/表语/定语.
3.还是老话,关系副词/关系代词的区别在于定语从句缺少的成分是什么.
如果缺状语,一定是关系副词.
且有如下变化公式：关系副词 = 介词 + 关系代词 = 介词 + 先行词.