Where were most people concentrated in ancient China, and why? in the river valleys, because they had fertile farmland along the coast, because it gave access to foreign trade in the highlands, because they had more space for herds of animals in the Plateau of Tibet, because it had fresh water from mountain runoff

in the river valleys, because they had fertile farmland along the coast

Explanation:

The history of ancient China can be traced as far back as around 2000 BCE. It was started with the Han Dynasty which lasted between 206 BCE to 220 CE.

During this early formation of ancient China and its civilization, most people concentrated in ancient China, around the Yellow River Region and the Yangtze Valley region.

These places were known to support the farming of millet and rice extensively due to their climatic conditions specifically its fertile land along the coast.

Hence, in this case, the correct answer is "in the river valleys, because they had fertile farmland along the coast."

a. theta waves. this is because the person is in a light sleep which means the brain waves would show these kind of waves.