Answer:
Nutrient pollution is the process where too many nutrients, mainly nitrogen and phosphorus, are added to bodies of water and can act like fertilizer, causing excessive growth of algae. Nutrients can run off of land in urban areas where lawn and garden fertilizers are used.
What finally enabled sturdy skyscrapers was sturdy steel, which is a mixture of metals and other elements, such as carbon, which is distinguished by low cost on one hand and strength and flexibility on the other.
People were able to specialize and trade, which made their city wealthier, stronger, and even more likely to make allies. :)
