What most likely brought about the formation of oxygen in the Earth's primitive atmosphere? 

A)  ozone producing prokaryotes 
B)  anaerobic prokaryotes 
C)  chemosynthetic bacteria 
D)  photosynthetic bacteria

When has a mineral resource been economically depleted? 

A)  When it costs more to mine the mineral than it is worth. 
B)  When 50% of the minerals have been mined. 
C)  When 80% of the minerals have been mined. 
D)  When there is no mineral left to mine.