The concept of escape velocity applies to objects as small as atoms and molecules. The earth's atmosphere is composed primarily of oxygen and nitrogen, and it contains practically no trace of hydrogen or helium, the lightest elements. All the atoms of a gas travel in random motion at very high speeds, and the less massive atoms and molecules travel at the greatest speeds. In fact, the lighter gases go so fast that their speeds are greater than the critical speed of 11.2 kilometers per second. Thus they have long ago left our comfortable home and are now wandering through space in search of a more hospitable gravitational field.
The moon has a weaker gravitation field than the earth. The escape velocity of an object on the moon is about 2.4 kilometers per second, and even the heavier gases have enough speed to escape the moon. This explains why we find no atmosphere there; any gases that may be released near the moon quickly escape.
Only when a ball exceeds 11.2 km' per second can it ______.

A. reach other planets
B. make a goal
C. escape from the earth
D. go upwards