The nitrogen cycle is an example of a cycle. Its processes include the conversion of atmospheric nitrogen into ammonia, which is called Choose... . Nitrogen is returned to the atmosphere in the process of Choose...

nitrogen fixation; denitrification

Explanation:

In a typical nitrogen cycle, the nitrogen in the atmosphere is converted into ammonia in a process known as nitrogen fixation. The ammonia is then converted into a usable form (nitrate or nitrite) for plants and other living organisms in a process known as nitrification. Nitrogen is returned to the atmosphere in a process known as denitrification.

Hence, the correct words to fill the gaps would be nitrogen fixation and denitrification.

you must remain stopped until the stop signal is retracted and the red lights are turned off. approaching traffic in the opposite roadway of a divided highway shall not be required to stop, even if the school bus has the stop-arm extended and the alternately flashing warning signal lamps on.