B
The other choices are nitrogen sources for plants.
Explanation:
Plants absorb nitrates (NO₃) and nitrites (NO₂) from the soil. Plants utilize these nitrogen sources which they assimilate to make plant proteins, amino acids and nucleic acids. Animals get their nitrogen sources from proteins that they acquire from feeding on the plants. These proteins are broken down into amino acids and used to make animal proteins. The amino acids also provide nitrogen source for which nucleic acids are formed.
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