An environmental study conducted in a particular city suggests that if a chemical plant emits more than 60 million tons of chemicals each year, the water supply will become contaminated beyond the point where filtration techniques can make it safe for drinking.

The correct answer is a. when they started making tools that will them in cultivating lands, they could raise crops and that made them attached to one place for a longer period of time. after that, they started domesticating animals and building their own permanent settlements that resemble the ones we have today.