it
这家工厂跟过去不同了我们老师说是that``that 不对吧这里表示的意思是同一家工厂，用it，前面还要加个主语whatThe factory is different from what it used to be.用that一般指代一个不可数名词，如：The weather of Beijing is different from that of Shanghai.不是的句子中没有what，原句就是the factory is different from ___used to be然后填代词that后来问他，他说因为used to be 不是句子是结构，that代指the +名词所以用它