The introduction of iodized salt in the 1920s dramatically reduced rates of in the united states.

The introduction of iodized salt in the 1920's dramatically reduced rates of goiter in the United States. This rapidly reduced the incidence of iodine deficiency. The United States and Switzerland were the first two countries to introduce the use of iodized salt to their country. 

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regarding the law, the 15th amendment was specifically designed thinking in the rights of black free men.

even though ratified on february 3, 1870, the promise of the 15th amendment would not be fully applied for a long time.poll taxes and literacy tests would allow southern states disenfranchise african americans. only in 1965 the voting rights for african americans would be actually able to vote in the south.

in 1965, president johnson signed the voting rights act, which aimed to overcome legal barriers that prevented african americans from voting as it was guaranteed by the 15th amendment to the u.s. constitution.