Question

21 which physiological reaction to stress helps to prepare the muscles for \fight
secretion of thyroxine
muscles tensing
decrease in lactic acid
increase in lactic acid
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Explanation:

When the body is under stress and preparing for "fight - or - flight", muscles tense up to be ready for physical action. Thyroxine secretion is more related to metabolism regulation in general. Lactic acid levels are related to muscle activity during and after exertion, not directly to the initial preparation for "fight - or - flight".

Answer:

Muscles tensing