C. because stimuli is how your body responds to something, and in this case, someone's heart rate increases WHEN their blood oxygen drop. Do you see how it responds? ;-)
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Nucleus is the control centre of the cell and manages all the cellular activities taking place inside the cell. It consists of four parts :-
- Nuclear membrane,
- Nucleoplasm,
- Chromatin material and
- Nucleolus.
The Chromatin material(Condensed - Chromosome) stores the cell's DNA which is transmitted to next generation.