<span>The theory of adjustment to aging that assumes older people are happier if they remain active in some way, such as volunteering or developing a hobby, is called activity theory.</span>
Because it is natural process
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Answer: It's an EXPERIMENT
Explanation: Experiment is done when we apply anything to a samples or conditions in other to study another sample or conditions.
Observation is done when, we study samples or conditions in it's natural way, without application of anything that may alter it's result.
Because the researcher has applied fertilizer to one side and did not apply to the other side, that means he has performed an experiment. Since he wasn't observing the two sample in it's natural way without any external application.
Some scientists believe that global warming is a myth. Due to whats in the atmosphere and what could be possible with it, they think its not likely.