The correct answer should be the industrial revolution. People started developing technologies that helped prevent diseases and this caused many more children to survive and many more people to live longer. The advances made in medicine are the key to rapid population growth, as well as era's like the world war 2 era after which came the baby boom generation where the population growth was at an all time high.
Provide jobs and faster production
Japanese feudalism made the citizens of Japan develop nationalism and after the fall of feudalism in Japan which were ruled by powerful shoguns Japan started the Meiji restoration which helped Japan open diplopmatic relationships with other countries thus helping them be modernized.
The correct answer is Warsaw. The Warsaw ghetto in Poland had the exact numbers that you said which means that there were more than 7 people per room. It was overcrowded and the people were not allowed to leave ever, and great monstrosities were committed to them during and after the ghetto time.
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