How could serfs move up from the bottom of the social hierarchy? They could prove themselves in battle.
They had to serve as a serf for a certain number of years.
They had to be freed by their lord.
They could marry into a higher level of the hierarchy.

the outcome of 1970 oil crisis in united states was that, they began to search ways on how to reduce oil.

in 1970 was a period when there were energy and oil crisis. those who were major centers for industrialization were forced to keep way from supplying of petroleum.

some of the result of this crisis was economic growth became stagnant because prices of oil surged.

due to the increased prices some countries like mexico and norway benefited.

europeans carried a hidden enemy to the indians: new diseases. native peoples of america had no immunity to the diseases that european explorers and colonists brought with them. diseases such as smallpox, influenza, measles, and even chicken pox proved deadly to american indians

serious tone and/or informal tone