As used in the fifth sentence of the fourth paragraph, the word "essentially" means ______.

A．a horizontal fault
B．a vertical fault
C．a more serious fault than the San Andreas fault
D．responsible for forming the Mississippi River

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