6. How do scientists determine when an era begins and when it ends?

Gold is not now being extracted from seawater. why not? a. it would be too disruptive to marine life to develop any marine mineral resources b. there is no gold in seawater c. we don't know how to extract minerals from seawater d. it would be too costly and difficult to separate the small amount of gold from the very large amounts of halite and other minerals

Why did the belgian parliament pass laws to decentralize its government? a. to increase trade with neighboring luxembourg b. to decrease the political power enjoyed by the flemish c. to enable belgium to join the european union d. to reduce tensions between belgium's two main ethnic groups

One disadvantage that darwin faces today is w. enemy attacks x. natural disasters y. lack of land bridge z. high population density