2.The Bush administration seems bent on satisfying the small but vocal group of Americans who oppose stem cell research under any conditions.
答案
1.【解析】该题考察which引导定语从句的用法。Past or present是插入成分，补充说明life on Mars.which 引导定语从句，修饰conditiona,which在从句中作方式状语，前面加介词under.
【句意】有关火星上过去和现在是否存在生命的更加确凿的答案，将向研究人员提供以下方面的珍贵数据，即行星在什么条件下能够导致最终产生生命的复杂化学反应。
2.【解析】该句为主从复合句。主句是the Bush administration seems bent on satisfying the small but vocal group of Americans,定语从句who oppose stem cell research under any conditions来修饰Americans.
【句意】少数人呼声不断，坚决反对出于任何目的的干细胞研究，布什政府似乎已经屈从于满足这些人了。
1.The road of life is long,but only a few steps are crucial especially during some years_______(当人年轻的时候)
2.April Fool’s Day is that special day of the year______(在这一天你可以捉弄别人)
答案
1.when people are young
【解析】该题考察when 引导定语从句的用法。when 在定语从句中充当时间状语，相当于“in which”
2.when you should play a joke on someone
【解析】该题考察when 引导的定语从句。When只能对that special day of the year这类表示时间的名词进行修饰。