Natural gas (methane (CH4)) is the cleanest burning fuel<span>, emitting the smallest amount of carbon dioxide of all the </span>fossil fuels<span> (coal, oil and natural gas).</span>
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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<span>If Noam chooses a filter, it might be more effective than a screen. After using it for a while, though, he will have to replace it, which could be expensive. A screen might be less effective, but it can be cleaned instead of replaced.</span>
