I don't understand how to answer this question:
Discuss the environment and habitat of a dog and of a flea that lives on it

Parasitism: The parasite (flea) lives off and harms the host (the dog)

Explanation:

well the answer is 2!

the nitrogen cycle provides nitrogen to the ecosystem from the atmosphere, ground and oceans. nitrogen is an important component of complex molecules such as amino acids and nucleotides, which lead to the creation of proteins and dna, the building blocks of all life. plants absorb nitrogen in the form of ammonium, nitrate ion and, on rare occasions, as amino acids. animals receive nitrogen necessary for biological processes from feeding on living or dead organic matter.