Tropical rainforest, the high mountains and wild rivers were some of the physical obstacles that the Incas faced. They had to build a complex network of roads to link the entire Inca empire. They had messenger stations along these roads so that they could relay their messages, and they built shelters along the way for travelers.
I believe A and C are correct.
Malaria
Dr. John Gorrie practiced medicine in Florida. He experimented with ways to prevent the spread of malaria. The summer of 1841 was a particularly bad summer for malaria infection. People had not yet realized that malaria was spread by mosquitoes. (That discovery didn't occur till the 1890s.) Dr. Gorrie, noting that malaria was unlikely to occur in colder climates, thought that perhaps cooling the air in the sick rooms of his patients could help them. He started with the idea of using a fan to blow air across a bucket of ice. A rudimentary beginning to air cooling and refrigeration ... but as they say, the rest is history!
<em>B. Soviet building of the Berlin Wall.</em>
Explanation:
After World War II ended, Germany got split up amongst the Allied powers. The United States, France, and Great Britain got West Germany, while the Soviet Union got East Germany.
The Soviet Union centered around communism, so East Germany had a communist government. Many Germans realized they did not want to obey and live under this government and wanted to flee. East Germans started leaving and moving to West Germany.
Over time, East Germany started to become fearful that too many people were leaving and decided they needed to do something about it. They built the Berlin Wall that was heavily guarded and hundreds of people ended up dying trying to get through.
This was the first cause of tension between the Soviet Union and President Kennedy. Both John F. Kennedy and Ronald Reagan gave speeches about the wall and how horrible it was.
