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summarize the negative feedback loop that keep the book balanced on your head. how did closing your eyes affect your ability to balance the book?

If I guess right, with your eyes open, it helps you with balance as you can see the environment, helping your brain calculate how to stand, but if you close your eyes, you are dependent on touch and other senses, which aren't viable when attempting to balance objects.

the glands which are associated with each description is given below:

pancreas - is an endocrine and exocrine gland that secretes juices with enzymes for digestion. salivary glands- with protection and digestion in the mouth. liver- secretes bile for digestion.   lachrymal glands - secrete tears to moisten eyes and protect them from particles and infection.   sweat gland -regulates water, salt, and temperature. oil glands - to form a protective barrier on the skin.   goblet cells - secrete mucus for protection.