先行词是the way,place,time,reason关系副、代词何时可省?

先行词是the way,place,time,reason关系副、代词何时可省?
为什么这句要加where:
We've all used thermometers to measure temperature but sometimes we need to measure temperature in places WHERE you can't put a thermometer.
语法说,先行词是the way,表示方式,可用that（可省）或in which来引导一个定语从句.如果先行词是时 间或原因,也可用that（可省）或when或why来引导定语从句.如：
（1）This was the reason that/why he was late yesterday.
（2）This is the last time that I shall give you a lesson.
这句是道改错题,还要加上where 是什么原因?
没表达清楚，我的意思是
that代替关系副词，那么我这道改错题为何还要加where?
有关that用法说明如下：that可以用于表示时间、地点、方式、理由的名词后取代when,where,why和"介词+ which"引导的定语从句，在口语中that常被省略，例如：
He is unlikely to find the place (that / where / in which) he lived forty years ago.他不大可能找到他四十年前居住过的地方。