Questions 11-20 refer to the following passage.

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

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

In line 8, the word "obliterated" means