How many galaxies are there in the observable universe? astronomy?

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

Explain: plate tectonics and people.

What is the difference between emigrate and immigrate? ?