I still remember the park（ ）we first met.A,that B,which C,wh

I still remember the park（ ）we first met.A,that B,which C,where D,when
这是一道定语从句.我不明白为什么不选B而选C.从句中成分应该是缺少宾语,用B才对啊,为什么选C?
还有这两句：
I will forget the days (when) I studied in the middle school.
I will forget the days(which) I spend in the middle school.
为什么第一个用when,第二个用which?