This is an experiment where the researcher manipulates one variable, and control/randomizes the rest of the variables. It has a control group, the subjects have been randomly assigned between the groups, and the researcher only tests one effect at a time.
Answer:
three types of stimuli—mechanical, thermal, and chemical; some endings respond primarily to one type of stimulation, whereas other endings can detect all types. Chemical substances produced by the body that excite pain receptors include bradykinin, serotonin, and histamine."
Explanation:
From edge
If you are implying one these choices not needed for survival it would be parents only when you can survive on your own (I'm not that sure but I hope this helped the slightest but)
