You are presented with the question "What in soil converts atmospheric nitrogen into a form usable by plants?" and the following answer choices: 
- compound of hydrogen oxygen
 - carbohydrates
 - single-celled organisms
 - excretory structures
 - peachleaf willow
 - members of their own species
 - It helps them survive
 - deoxyribonucleic acid

Now knowing that bacteria in soil convert atmospheric nitrogen into a form usable by plants and bacteria are single-celled organisms, choose the best answer.


 

   single-celled organisms      