Microplastics have become a common source of pollution across the Earth — they have settled in the deep sea and on the Himalayas, stuck inside volcanic rocks, filled the stomachs of seabirds and even fallen in fresh Antarctic snow.
请选择正确的选项填入空白处：Microplastics ___ a common source of pollution across the Earth — they have settled in the deep sea and on the Himalayas.