Fact 1: Bacteria in soil convert atmospheric nitrogen into a form usable by plants.

Fact 2: Bacteria are single-celled organisms.

Given the two facts above, answer the question "What in soil converts atmospheric nitrogen into a form usable by plants?" with the following options: 
- compound of hydrogen oxygen
 - carbohydrates
 - single-celled organisms
 - excretory structures
 - peachleaf willow
 - members of their own species
 - It helps them survive
 - deoxyribonucleic acid


 

   single-celled organisms      