In this section you will find after each of the passages a number of questions or unfinished statements about the passage, each with four (A. B. C and D) suggested answers or ways of finishing. You must choose the one which you think fits best. Then mark the corresponding letter with a single bar across the square brackets on your Machine-scoring ANSWER SHEET.

　　Questions 51-56 are based on the following passage.

　　As viewed from space, the Earth's distinguishing characteristics are its blue waters and white clouds. Enveloped by an ocean of air consisting of 78% nitrogen and 21% oxygen, the planet is the only one in our solar system known to harbor life. Circling the Sun at an average distance of 149 million km (93 million miles), the Earth is the third planet from the Sun and the fifth largest planet in the solar system. Its rapid spin and molten nickel-iron core give rise to an extensive magnetic field which, coupled with the atmosphere, shields us from nearly all of the harmful radiation coming from the Sun and other stars. Most meteors burn up in the Earth's atmosphere before they can strike the surface. The planet's active geological processes have left no evidence of the ancient pelting it almost certainly received soon after it was formed. The Earth has a single natural satellite —— the Moon.

　　51. Approximately how much of the Earth's atmosphere is nitrogen?

　　A. One-fourth

　　B. One-half

　　C. Three-fourths

　　D. All of it

　　52. Which of the following helps to create the Earth's magnetic fields?

　　A. Its blue waters

　　B. Its nitrogen atmosphere

　　C. Its molten metal core

　　D. The Moon

　　53. What two factors help protect the Earth from radiation?

　　A. Magnetic field and atmosphere

　　B. Rapid spin and molten iron-nickel core

　　C. The Sun and the Moon

　　D. Blue waters and white clouds

　　54. Why does the Earth show almost no signs of having been hit by numerous meteors in the past?

　　A. Humans have built over most of the craters.

　　B. Most meteors fell into the ocean and not on land.

　　C. The Earth's magnetic field repelled most meteors.

　　D. The Earth's natural geologic activity has eliminated most traces.

　　55. The main idea of this passage is that

　　A. there are life-supporting characteristics on the Earth.

　　B. The Earth is predominantly water.

　　C. The Earth has no common characteristics with other planets.

　　D. The Earth is the only planet with a moon.

　　56. This selection leads one to believe that

　　A. The Earth never gets hit by meteors.

　　B. The Earth always gets hit by meteors.

　　C. The Earth was hit by meteors some time in the past.

　　D. The Earth may be bombarded by meteors in the near future.

　　Questions 57-62 are based on the following passage.

　　Since life began eons ago, thousands of creatures have come and gone. Some, such as the dinosaurs, became extinct due to naturally changing ecologic conditions. More recent threats to life forms are humans and their activities. Man has drained marshes, burned prairies, dammed and diverted rivers. Some of the more recent casualties of man's expansion have been the dodo, great auk, passenger pigeon, Irish elk, and Steller's sea cow. Sadly, we can no longer attribute the increasing decline in our wild animals and plant species to "natural" processes. Many species are dying out because of exploitation, habitat alteration or destruction, pollution, or the introduction of new species of plants and animals to an area. As mandated by Congress, protecting endangered species, and restoring them to the point where their existence is no longer jeopardized, is the primary objective of the U.S. Fish and Wildlife Service's Endangered Species Program.