题目内容：

Be sure to practise all four language skills -__,reading,writing ,listening and speaking
为什么要用that is 不用they are ,this is 讲一下做法

优质解答

that is是that is to say的简略形式,是一种固定搭配,做插入语,意思是“就是说”.作为独立成分,与主句主语没有主谓关系.