定语从句

We will strengthen ongoing and ex post oversight ofitems for which approval has been cancelled or delegated to lower-levelauthorities.

老师①这句话的forwhich中的for是如何出来的呢？②用of可以吗？为何？

for which.....，后面是个定语从句，介词+关系代词结构，for表缘由，which指代先行词post oversight of items，不可以用of，of不可以表缘由。