Polytheism is a religion defined by the belief in more than one God. In the ancient world almost all religions were polytheistic—such as the Greek and Roman religions, Egyptian, Norse and so on; however, since the rise of Judaism, then Christianity and Islam, monotheistic religions (the belief in one God) now largely dominate.

Cultural diffusion is a term used to describe the spread of one culture’s language, ideas, or principles to other cultures, where it is then adopted and embraced. In the last century or two the English language has spread rapidly around the world and is now the second language of a large proportion of the global population.

The original Native American population is believed to have crossed into North America via a land bridge in the modern-day Bering Strait. So they would have crossed from what is now Northern Russia into Alaska and then rapidly worked there way (over many generations) down throughout modern North and South America.

A refugee is someone who has either been forced to leave or else has fled some disaster—famine, poverty, genocide, religious warfare, etc. A refugee leaves his or her own country and heads for a willing country believed to be safer. Different countries around the world have widely varied practices when it comes to accepting refugees. The diaspora is the name given to a community of displaced people, e.g. the Jewish Diaspora.

The phrase “God, glory, and gold” is often used to describe the motivations behind European conquest and settlement of the Americas. It is most frequently used to describe the first wave of European colonialism under the Spanish. It is a shorthand term to illustrate how the Spanish were motivated to colonize the Americas in order to spread Christianity ("God"), gain personal fame and notoriety ("glory"), and become immensely wealthy by exporting gold and silver back to Europe ("gold").

Xenophobia is an important term and important trend in human and American history. Xenophobia is the fear of foreigners, and it is a motivating factor in many of the worst instances of violence and atrocities in human history. It can also be a primary motivating factor in less dramatic, but no less important, circumstances, like limits on immigration, racism, tariffs, civil unrest and so on.

The Baby Boom period lasted for about twenty years after the end of World War Two. It was marked by a massive increase in the size of families and the population of the country as a whole. It was caused by several factors, including a much higher standard of living, the relatively low level of birth rates during World War Two, and decreased infant mortality rates. Another contributing factor was the GI Bill. The GI Bill provided loans and grants to returning veterans from World War Two to help them buy houses and have access to higher education. The only answer choice that did not directly contribute to the Baby Boom generation was the Civil Rights Movement. Firstly, this did not really begin in earnest until the mid-1950s, second of all it was a movement to improve social rights and not an economic or health based movement so cannot be expected to have much effect on the birth rate.

Prior to the Industrial Revolution, the population of the world was less than one billion. It would pass one billion early in the nineteenth century, but would not reach two billion until early in the twentieth century. Since the massive improvements in public health, medicine, technology, and farming practices of the twentieth century, the world population has risen to over seven billion. So the correct answer is easily the twentieth century. 

More than half the people on the planet live in the continent of Asia: the majority of these living in China and India. Of the more than seven billion people on Earth, at least four billion live in Asia. 

More than half the people in the world live in Asia. After Asia, the next highest is Africa, which is the only other continent to have more than a billion people living there. After Africa comes Europe, then North America, then South America. The population of Europe is projected to remain approximately the same for several decades, so by the end of the twenty-first century it will have likely been overtaken by all the other continents, except Oceania and Antarctica.