what factor determines that an oxygen atom can form two covalent bonds while carbon atom can form four

rocks and weathering-matching

a chain link fence because there so tiny even my albino donkey can't get out

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.