What are two experiments that would allow you to test that increased predation by killer whales was the cause of the sea otter decline?

The key components of a good experiment: a control (something to which to compare the treatment), replication (do it more than once), and consideration of confounding factors (what might cause differences other than what you manipulate in your experiment?) I am able to figure out the first one (comparing presence... show more The key components of a good experiment: a control (something to which to compare the treatment), replication (do it more than once), and consideration of confounding factors (what might cause differences other than what you manipulate in your experiment?)

I am able to figure out the first one (comparing presence of killer whales to sea otter decline) but the second I cannot. Please help!
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