prep.+which/prep.+whom中介词的选择：
一先，二谓语，三意义
the occasion when/on which
the place where/at which
situation/case where/in which
point where/at which
activity where/in which
若先行词中有数字，通常后面的定语从句用of which/of whom
it is high/about time that...should+do/did
it/this/that is the first time that...have done...
it/this/that was the first time that...had done...
