一个独立主格 The bus was crowded with passengers going home from m

一个独立主格 The bus was crowded with passengers going home from market,most of ________
The bus was crowded with passengers going home from market,most of ________ carrying heavy bags and baskets full of fruit and vegetables they had bought there.
A.them B.who C.whom D.which
most of them carrying...为独立主格结构,相当于 and most of them were carrying...,也可改为：most of whom were carrying.
我是想问 为什么相当于and most of them were carrying..,这个were carrying 是表示过去进行时?were 是什么用法 那前面也没交代是进行时啊