To colacat: vendor 也有“小商贩”的意思。因此,我认为:many of them children 是插入语,补充修饰 vendors and beggers;其中,many of them 是修饰children的。后面的who mob cars... 中who定从修饰的是vendors and beggars。请指教~
C选项是基本的主系表结构, 如果把它拓展开就更明显了:
A phenomenon is so common that there are waves of venders and beggars. 在这里waves是phenomenon的表语,表语不能决定主语的动词形式。而many of them children是独立主格,作插入语。