一道高一英语单选题,来帮帮忙吧

一道英语选择题写题时遇一纠结题，

What do you think of teaching? -I find it fun and challenging。It is a job __you are doing something serious but interesting。 A where Bwhich Cwhen Dthat 本题选A。 理由：B和D为关系代词，在定语从句中要做主语或宾语，而后面you are doing something serious but interesting是一个完整的句子，并不缺这两个成分。 when不能选，因为when引导时间状语从句，或在定语从句中说明时间，本题明显不合...全部

  What do you think of teaching? -I find it fun and challenging。It is a job __you are doing something serious but interesting。
   A where Bwhich Cwhen Dthat 本题选A。 理由：B和D为关系代词，在定语从句中要做主语或宾语，而后面you are doing something serious but interesting是一个完整的句子，并不缺这两个成分。
   when不能选，因为when引导时间状语从句，或在定语从句中说明时间，本题明显不合适。 所以选A，alexpascal分析的就很好。 关于句子成分：It is a job是主系表结构，where引导的定语从句修饰前面的job,做定语。
  整句意思：教学是一份这样的工作，做这份工作既认真又有趣。 。收起