TASK DEFINITION: Given a scientific question, generate a correct answer to it.
PROBLEM: The nitrogen that enters living systems by nitrogen fixation is successively converted from organic nitrogen back into nitrogen gas by what?

SOLUTION: bacteria

PROBLEM: Neutron stars are the corpses of left over what?

SOLUTION: supernova explosions

PROBLEM: Which veins return oxygen-rich blood from the lungs to the heart?

SOLUTION:
pulmonary