6. the average copper ore mined in 1900 was five percent copper by weight. copper ore mined in 1990 was approximately 0.5 percent copper by weight. what factors could account for this difference? "

The volume of miners and industries using copper in the 20th century presented an exponential growth. Mining tends to extract the highest grade resources first, once they are the most profitable. With the intense activity achieved during the last century, the higher grade non renewable sources were quickly depleted, forcing the industry to start exploring lower grades sources to satisfy the demand.

It is also noteworthy that mining copper of higher quality causes more damage to the environment, having its protocols and restrictions consequently addressed by the legislation. Therefore the costs of mining 5% copper might be outweighed by a higher margin of profit from 0.5% copper which also is less damaging to the environment.

Rapa Nui era nativa da ilha de Páscoa.

Explicação:

Rapa Nui era o nativo da ilha. Esta ilha foi descoberta em 1722 pelo holandês Jacob Roggeveen. Esta ilha tem uma grande importância devido à presença de metais preciosos como cobre, minério de ferro, nitratos e molibdênio etc. Essa ilha também é uma grande fonte de madeira, mas para manter sua beleza natural, as árvores não devem ser cortadas para madeira. Para manter a beleza natural da ilha, os recursos devem ser extraídos sem o uso de máquinas que danifiquem o meio ambiente e causem poluição.