If single-celled organisms in soil convert atmospheric nitrogen into a form usable by plants, then what in soil converts atmospheric nitrogen into a form usable by plants?

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

 

   single-celled organisms       