What was the major factor contributing to the explosive population growth in More Developed Countries (MDCs) during the 19th and early 20th centuries?

The answer is A. The diffusion of the Industrial Revolution.

The Industrial Revolution began in England in the late 1700s and lasted through all of the 19th century. By the 1850s, the Industrial Revolution made its way to the rest of Europe. With the advent of the Industrial Revolution, the quality and standard of life also increased. This rise in the quality of life and urbanization made people lead healthier lives and mortality rates dropped in all of Europe. In addition to this, the advent of the Industrial Revolution created a labor economy and the requirement of human labor increased with the rise of factories. The greed for money meant that married couples among the lower classes bred more children so that they could grow up and start working. This attitude was found only in the worker's families whereas the richer families bred fewer children where the concept of nuclear families begun to develop.

Throughout history, the population in certain areas have spiked due to certain reasons. The population also decreased rapidly due to certain events. For instance, the plague was a deadly and very contagious disease in Europe. Many people died. When many farmers died, the population decreased.

Those that did not die from the plague may have died from not having access to doctors or other people who helped them survive. On the other hand, the population grew as a result of the Industrial Revolution. Families were able to grow due to being able to have their children get jobs.

This event took place in the 19th and early 20th centuries. However, this only applied to more developed countries since they would have access to creating factories for these families to work in.

Prior to the Industrial revolution in the United States and in England, there were really only two economic classes of people. The people were either very rich or very poor. As a result, most people struggled in their countries to make ends meet and not struggle, but it was hopeless.

When the Industrial Revolution took place, many new inventions were created like the steamboat and the spinning jenny. These allowed for workers to be hired and make a relatively decent wage. This began a middle class.

Many people emigrated to these countries and then sent money back to their own countries which helped these countries to expand in population. More people were born to work these jobs in the factories.