How do plants obtain nitrogen?
a. they remove it from the air.
b. they absorb it through their roots.
c. they fix it in their leaves.
d. they produce it in their stems.

They absorb it through their roots. B.

Explanation:

Plants take nitrogen from the soil by absorption through their roots as amino acids, nitrate ions, nitrite ions, or ammonium ions. Plants do not get their nitrogen directly from the air. Plants get the nitrogen that they need from the soil, where it has already been fixed by bacteria and archaea.

B. They absorb it through their roots.

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