Answer:
It simply refers to the word their.
Explanation:
Producers are organisms that get their energy directly from the Sun. Their cells are able to turn sunlight into food through a process called photosynthesis. In photosynthesis, producers combine carbon dioxide, water, and sunlight to produce oxygen and sugar (their food).
The clause <em>when we can rest </em>is a subordinate/dependent clause, meaning that it cannot stand on its own - it has to be a part of a larger, independent clause, as is the case here.
