Answer the following question given this paragraph: 

Unlike energy, matter doesn’t need to be constantly added to ecosystems. Instead, matter is recycled through ecosystems. Water and elements such as carbon and nitrogen that living things need are used over and over again.


Q: Unlike energy, what doesn’t need to be constantly added to ecosystems because it is recycled through ecosystems?

 Choices:

- water

- nutrients

- vapor

- matter

A:
matter
