答案是A.
1. 句子主干是Everything must/should be done right now.
2. 句子主语everything被一个定语从句所修饰,即everything that should be done.此题因为先行词是不定代词everything, 关系代词又要在定语从句中充当主语,所以关系代词只能使用that.排除C；
3. 结合整个句意和复合句结构,可以排除B.如果选B,一则定语从句中should be后面无Vpp；二则主句的谓语变成了must done be done了.
还可以排除D.如果选D,定语从句中的谓语结构是should be,后面也缺少Vpp；另外意思也是重复的,从句是should be,主句也是should be.
4. 最终句子为：Everything (that should be done) must be done right now.
翻译：该要做的每一件事情必须马上做好!