托福英语指代题Glaciers may form in permanently cold areas,and these
托福英语指代题
Glaciers may form in permanently cold areas,and these slowly moving masses of ice cut out valleys,carrying with THEM huge quantities of eroded rock debris.
them指代谁?我觉得指代valleys特顺,但是他说是masses of ice,为什么?
顺便请帮我分析一下句子结构,我不明白逗号后面的结构意思,