Explain why oil is so important to these countries, US, Saudi Arabia, Russia, Canada, China, Iraq.

The United States' dependence on oil has long influenced its foreign policy. This timeline traces the story of U.S. oil development, and the resulting geopolitical competition and environmental concerns, in more than forty milestones. The three major periods include the rise of oil as a commodity, beginning in 1850; the post-WWII age of geopolitical competition; and the current era of deregulation and diversification.

radioactive dating, also known as  radioisotope dating  is a process that in  estimating the approximate  age  of  rocks. these data can be analyzed from  the studies dealing with the  decay of the radioactive elements that are present in these rocks. in nature, there are a few types of materials or atoms which are quite unstable and they spontaneously decay or breakdown to form other kinds of atoms.

2018 means 2018 ce or common era

there have been other eras too including bce, which included way more years.