​Choose the correct answers to complete the following short passage.

We believe the Earth is about 4.6 billion years old. At percent, we areforced to look to other bodies in the solar system for hints as to what theearly history of the Earth was like. Studies of our moon, Mercury, Mars, andthe large satellites of Jupiter and Saturn have provided ample evidence thatall these large celestial bodies had formed. This same bombardment must haveaffected Earth as well. The lunar record indicates that the rate of impactsdecreased to its present low level about 4 billion years ago. On Earth,subsequent erosion and crustal motions have obliterated the craters that musthave formed during this epoch.

Scientists estimate the Earth’s age by measuring the ratios of variousradioactive elements in rocks. The oldest Earth’s rocks tested thus far areabout 3 1/3 billion years old. But no one knows whether these are oldest rockson Earth. Tests on rocks from the moon and on meteorites show that these areabout 4.6 billion years old. Scientists believe that this is the true age ofthe solar system and probably the true age of the Earth