The best and most correct answer among the choices provided by the question is the third choice (3) , conduction occurs when two bodies has connected to transfer heat.
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The development of the population policies in the 20th century were due too multiple factors, the most important of which were:
- Fertility rates were starting to lower down, especially in the ''western world'' in the second half of the 20th century.
- Increased demand for labor force.
- The World Wars, especially the second one caused huge migration movements.
- Millions of people from less developed countries were trying to migrate to more developed countries in search of a better life.
