student shoudn'tbe given so difficult a problem (as) they can not work out.括号里为什么不用which?指代problem

student shoudn'tbe given so difficult a problem (as) they can not work out.括号里为什么不用which?指代problem
student shoudn'tbe given so difficult a problem (as) they can not work out.括号里为什么不用which?
指代problem

student shoudn'tbe given so difficult a problem (as) they can not work out.括号里为什么不用which?指代problem
as 这里是因为
原因状语从句 which不能引导原因状语

表原因啊