这是什么从句？填that 还是where？

从整体上看，这是一个强调句。句型是it was....that...是与第二空的that构成的。
但是其中还包含了定语从句，也就是第一空的that引导的。
可以翻译为：就是在Jone教授的实验室里，这个实验被执行的。

It was in the lab （ that was in the charge of Professor Jone） that the experiment was carried out.打括号部分是由that引导的修饰lab的定语从句，整个句子属于强调句型。

整个句子是强调句型，但是套了定语从句

分开来这么看：It was in the lab ( that )the experiment was carried out，这是整个句子的大结构

It is (was)+强调部分+that/whom/who+句子其它部分构成强调句型，如：
It was he taht/who bought a camera yesterday

之前的这个部分： the lab ( )was in the charge of Professor Jone 是个定语从句，用“that”作为定语从句的引导词。

以上，希望能有帮助~~

第一个that是定语从句 that was in the charge of professor Jone。先行词是lab

第二个that是强调句。

IT was in the lab (that was in the charge of Professor Jone) THAT the experiment was carried out.

it is ....that 这个是强调句型。所以第一个空就选that。第二个空的位置是定语从句，修饰Professor Jone ,修饰人的的先行词就只能用that 或者who.所以就是that.所以就选A.