The nitrogen cycle is introduced with the observation that getting nitrogen into the living world is:
AEffortless
BImpossible
CInstantaneous
DDifficult
Answer & Solution
Correct answer: D. Difficult
1. Each biogeochemical cycle has its own bottleneck.
2. Carbon is the second most abundant element in living organisms.
3. The nitrogen cycle is introduced separately from the carbon cycle.
4. Getting nitrogen into the living world is difficult.
5. That difficulty is what makes nitrogen fixation so important.
_Source: OpenStax Biology 2e (CC BY 4.0), Ch 46 'Ecosystems', sections 46.1-46.3_
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