高考英语两道单选题对比~~~~~~~~~ 求解

题目

高考英语两道单选题对比~~~~~~~~~ 求解
1.He has written three books on computer networking,_____one became a best-seller and was published in twelve languages.
答案是of which
2.Mr.Green has two sons,one of____working as a bus driver now.
答案是them
请问这两道题有什么区别?

答案

第一道题是定语从句,先行词是book,which做连接词,后半句构成一个从句
第二道题是非谓语,逗号后面的半句不是句子,因为是动名词working而不是动词work,不是句子就不需要连接词,所以是one of them