As used in the first sentence of the fourth paragraph, the word "essentially" means_____.

A．horizontal faults are more dangerous than vertical faults
B．Vertical faults are more dangerous than horizontal faults
C．Earthquakes occur only around fault areas
D．California will break into pieces by an eventual earthquake

A．the New Madrid fault in Missouri
B．the San Andreas and the New Madrid faults
C．the causes of faults
D．current scientific knowledge about faults