most of them还是most of which

most of them还是most of which
 
请看看下面这道题，答案是 most of them 还是 most of which：
The growing speed of a plant is influenced by a number of factors, ______ are beyond our control.
A. most of them       B. most of which       C. most of what       D. most of that http://yingyu.chazidian.com
【分析】答案为B。由于两个句子之间没有并列连词 and，故可排除 A 和 D；又因为 what 不能用于引导定语从句，所以 most of what 也可排除。此处的 most of which 相当于 and most of them，所以若在选项 A 的前加上 and，则也是对的。
比较：
Some managed to vote, but most of them didn’t. 有些人设法投票了，但多数人都没有。
The buses, most of which were already full, were surrounded by an angry crowd.  公共汽车大多数都已经挤满了人，它们被愤怒的人群包围着。