The four limitations are
1.It's often impossible to repeat trials on the same subjects.
2-Subjects may report an inaccurate medical history.
3-It can be difficult to control all possible variables.
4-It's impossible to come up with testable scientific questions for human subjects.
Research involving human beings is ultimately necessary for improvements in human health and welfare. Any results about healthy physiology, illness causes, treatment effectiveness, learning, or behavior must be verified by carefully controlled investigations involving human participants. Sadly, not all human research have been valid and beneficial. It is possible to harm people in the name of research. The most well-known instances happened in Nazi Germany. Investigations conducted after the war found several crimes, including experiments in which participants were submerged in extremely cold water to determine how long it would take them to pass away from hypothermia.
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Answer:Towhen two waves if nearly same amplitude and same wavelength interfere with each other that time a new wave is formed with a different amplitude this phenomena is known as superposition. If two waves interfere with each other in same phase that time amplitude of resultant wave wave is greater than the component waves and this type of interference is known as constructive interference. And when waves suprpose in opposite phase that time amplitude of resultant wave is less than the component waves and this type of interference is known as destructive interference.
Example: light waves, water waves, sound waves etc
Alkaline earth metals
Group 13 aka the boron family is categorized as alkaline earth metals
"The proportion of Neanderthal-inherited genetic material is about 1 to 4 percent [later refined to 1.5 to 2.1 percent]. It is suggested that 20 percent of Neanderthal DNA survived in modern humans, notably expressed in the skin, hair and diseases of modern people.Extinction of a particular animal or plant species occurs when there are no more individuals of that species alive anywhere in the world - the species has died out. This is a natural part of evolution. But sometimes extinctions happen at a much faster rate than usual.
