The word "These" in the second pa...

B

- Hướng dẫn giải

Phương pháp giải:

Giải chi tiết:

B                

“these” được thay thế cho “the components in biogeochemical cycles” trong câu phía trước: “As the Earth developed, the concentration of these pollutants was altered by various chemical reactions; they became components in biogeochemical cycles.”