Q: John was studying about the history of Sahara desert. He was amazed to find that ten thousand years ago there were plenty of rainfall in that region. He labeled that era as era A. Then he found that rainfall deceased in the region about eight thousand years ago. He labeled that era as era B. He was wondering how that change in rainfall might have helped emerge the Egyptian civilization. In which era new surfaces would not be exposed, era A or era B?
A: era B

Q: There are two hydroelectric dams in North Dakota. Both of them has large lakes or reservoirs, reservoir A and reservoir B. Since North Dakota has one the lowest unemployment rates in the country, it's hard to find an engineer to regulate and maintain the reservoirs. This year reservoir A could hire an engineer, but reservoir B failed to find an engineer. As a result, there is no one to regulate reservoir B. The engineer for reservoir A let the water out in the fall. But there was no one to do that for reservoir B. Which reservior would most likely not have excellent water quality, reservoir A or reservoir B?
A: reservoir B

Q: John is learning about different states of matter. So far, he recognized three states of matter, state A, state B, and state C. State A is in solid state. State B is in liquid state, and state C is in gaseous state. He needs to learn more about their individual characteristics. In which state the particles would be farther, state B or state C?
A: state C

Q: Stargazers across the world wait for 2020 when newly discovered very bright Neil's comet would start to pass through the inner solar system On December 21, 2020. After December 27, 2020 the comet would leave the inner solar system. Like many other enthusiasts Ken bought a telescope for the event. His friend Mike is interested in that event too, but is too busy to buy a telescope. On which date volatile materials within the comet would start to vaporize?
A:
December 21, 2020