I can never forget the days_____we worked together and the days____we spent together.

选择D
we worked together (in) the days,因为句子里面没有介词IN,所以填when或者in which
we spent the days together,不需要介词,所以填which
把句子缺少的部分还原看下就好了为什么这样还原不行呢？we worked together on the days所以填on whichwe spent together (in) the days所以填when在这些日子里一起工作，所以用in，on的话是具体日子，星期一星期二这样的spend time （in） doing sth 花费时间做某事spend time on sth 花费时间在某事上这个词组里面的时间可以换成精力，金钱等