According to the passage how many miles long was the trench that became known as the Grand Canyon

A．its shapes are constantly changing
B．it contains approximately 14 million tons of rock
C．its eruptions have increased in recent years
D．it is being eroded by toxic waste and pollutants

A．patterns of erosion in different mountain ranges
B．forces that made the Grand Canyon
C．the increasing pollution of the Colorado River
D．the sudden appearance of the Grand Canyon