Scientists discover that the atmosphere of both the Earth and Mars contains nitrogen. They then conclude that the atmospheres of all planets must contain nitrogen.

answer; /// i believe the correct answer is (reactants);

they had the law tat factories that produces smoke cannot be built near residential buildings.

building of proper sewage system was done to ensure that the waste is carried away from the humans so that they do not get diseases from it.

also they built water systems to provide water in those areas which are drought prone.