书上说是定语从句.it在不知道是谁的情况下可以表示人,然后that就是连接词

还是Who is it that won the first prize in the later National Maths Competation?这句话吧.再分析一下：1.Who is it ?为主句,that won the first prize in the later National Maths Competation为that引导的定语从句修饰it. 2.若看作强调句型把题中的is改为was,即Who was it that won the first prize in the later National Maths Competation? 把此句中的was it that （强调结构）去掉之后成为Who won the first prize in the later National Maths Competation? 句意完整.所以是强调句型.
总结,若句子用is是定语从句,若用was则是强调句型,即be动词的时态是关键.这是容易理解出错的地方,可能也是出题者的意图所在.
以上是个人的目前看法,不知你是否赞同.

什么东西?看不懂.///
应该是说在先行词既有人又有物时,关系代词用that,
比如There are 2 boats and 3 people that I saw in the park in this picture.
这里的先行词是2 boats and 3 people 既有人又有物,所以定语从句I saw in the park 用that做关系代词