What is the initial stage of all stars? nebula black hole supernova white dwarf

Nebula

Explanation:

Nebula is a stellar nursery. Stars take birth in the molecular cloud of dust and gases -Nebula.

This molecular condenses and forms a core which spins and gathers even more mass and becomes bigger. This process increases temperature and pressure at the center which ultimately starts nuclear fusion reaction giving birth to a star which liberates energy in the form of heat and light.

Black-hole is the end stage of a massive star.

Supernova is a stage which precedes formation of a neutron star or a black-hole for a massive star.

White dwarf is the end stage of an average mass star like sun.

explanation:

b,bacteria convert nitrates

into nitrogen gas.

d,plants and animals take in

and use nitrogen compounds.

e,bacteria break down waste    

products and return nitrogen  

to the soil.

a,bacteria convert atmospheric

nitrogen to ammonia.

c,bacteria convert ammonia

to nitrites and nitrates.