Which of the following best describes the "observation" referred to in the passage? A. The rocky planets were created by the rapid gravitational collapse of a dust cloud. B. Certain features on the Moon's surface are impact craters caused by collisions with objects such as planetesimals. C. The rocky planets were formed by a slow accretion of cosmic dust into increasingly larger bodies. D. The number of objects colliding with the Moon appears to have been high for a while and then rapidly diminished. E. There are far fewer planetesimals in existence today than there were about 4.5 billion years ago.