the bus ,most of ----full of passengers were trapped in thet

the bus ,most of _them___full of passengers were trapped in thetraffic jam yesterday afternoon.
因为整个句子只有一套主谓宾,如果用which构成定语从句的话,句子就没有谓语和宾语了.所以这能用them.
再问： 如果填most of which full of passengers，做the buses的定语不行吗 ，为啥就得填most of them，谢谢，
再答： most of which full of passengers 不是定语从句，语法上有错，定语从句中缺少谓语