By Earth's history standards, the Grand Canyon is ________.
A) not important
B) as old as the Earth
C) very old
D) youthful
E) becoming a first-order landform
Question 2
The present estimate of the Earth's age is about ________ years.
A) 50 million
B) 500 million
C) 2.8 billion
D) 4.6 billion
E) 2.8 trillion