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critical

mastery

/ˈkɹɪt.ɪ.kəl/adjnoun

card 1 of 10 · context meaning

Which meaning of 'critical' best matches its use in the sentence?

Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis. The critical component of the photosynthetic system is the 'water-oxidizing complex'.