According to the passage, what is "black gold"

A．To indicate the extent of United States mineral wealth.
B．To argue that gold was more valuable than oil.
C．To describe the mood when oil was first discovered.
D．To explain the need for an increased supply of gold.

A．Oil Refining: A Historical Perspective.
B．Kerosene Lamps: A Light in the Tunnel.
C．The California Gold Rush: Get Rich Quickly.
D．Private Property: Trespassers will Be Prosecuted.