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Questions 21-30 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...

Questions 1-10 refer to the following passage. The most familiar speleothems (from the Greek word spelaion for the cave and thema for deposit), the decorative dripstone features found in caves, are stalactites and stalagmites. Stalactites hang downward from the ceiling of the cave and are formed as drop after drop of water slowly trickles through crack on the cave roof. Stalagmites grow upward from the floor of the cave, generally as a result of water dripping from an overhead stalactite. A column forms when a stalactite and a stalagmite grow until they join. A “curtain” or “drapery” begins to form on an inclined ceiling when drops of water trickle along a slope. Natural openings on the surface that lead to caves are called sinkholes. or swallow holes. Streams sometimes disappear down these holes and flow through the cavern. Rivers may flow from one mountain to another through a series of caves . Some caverns have sinkholes in their floors. Water often builds up a rim of dripston...