If the salinity of ocean waters is analyzed, it is found to vary only slightly from place to place. Nevertheless, some of these small changes are important. There are three basic processes that cause a change in oceanic salinity. One of these is the subtraction of water from the ocean by means of evaporation —— conversion of liquid water to water vapor. In this manner, the salinity is increased, since the salts stay behind. If this is carried to the extreme, of course, white crystals of salt would be left behind.

　　The opposite of evaporation is precipitation, such as rain, by which water is added to the ocean. Here the ocean is being diluted so that the salinity is decreased. This may occur in areas of high rainfall or in coastal regions where rivers flow into the ocean. Thus salinity may be increased by the subtraction of water by evaporation, or decreased by the addition of fresh water by precipitation or runoff.

　　Normally, in tropical regions where the sun is very strong, the ocean salinity is somewhat higher than it is in other parts of the world where there is not as much evaporation. Similarly, in coastal regions where rivers dilute the sea, salinity is somewhat lower than in other oceanic areas.

　　A third process by which salinity may be altered is associated with the formation and melting of sea ice. When seawater is frozen, the dissolved materials are left behind. In this manner, seawater directly beneath freshly formed sea ice has a higher salinity than it did before the ice appeared. Of course, when this ice melts, it will tend to decrease the salinity of the surrounding water.

　　In the Weddell Sea, off Antarctica, the densest water in the oceans is formed as a result of this freezing process, which increases the salinity of cold water. This heavy water sinks and is found in the deeper portions of the oceans of the world.

　　4l. What does the passage mainly discuss?

　　(A) The elements of salt

　　(B) The bodies of water of the world

　　(C) The many forms of ocean life

　　(D) The salinity of ocean water

　　42. The word "this" in line 5 refers to

　　(A) ocean

　　(B) evaporation

　　(C) salinity

　　(D) crystals

　　43. According to the passage, the ocean generally has more salt in

　　(A) coastal areas

　　(B) tropical areas

　　(C) rainy areas

　　(D) turbulent areas

　　44. All of the following are processes that decrease salinity EXCEPT

　　(A) evaporation

　　(B) precipitation

　　(C) runoff

　　(D) melting

　　45. Which of the following statements about the salinity of a body water can best be

　　inferred from the passage?

　　(A) The temperature of the water is the most important factor.

　　(B) The speed with which water moves is directly related to the amount of salt.

　　(C) Ocean salinity has little effect on sea life.

　　(D) Various factors combine to cause variations in the salt content of water.

　　46. The word "altered" in line 16 is closest in meaning to

　　(A) determined

　　(B) changed

　　(C) accumulated

　　(D) needed

　　47. The world "it" in line 18 refers to

　　(A) sea ice

　　(B) salinity

　　(C) seawater

　　(D) manner

　　48. Why does the author mention the Weddell Sea?

　　(A) To show that this body of water has salinity variations

　　(B) To compare Antarctic waters with Arctic waters

　　(C) To give an example of increased salinity due to freezing

　　(D) To point out the location of deep waters

　　49. Which of the following is NOT a result of the formation of ocean ice?

　　(A) The salt remains in the water.

　　(B) The surrounding water sinks.

　　(C) Water salinity decreases.

　　(D) The water becomes denser.

　　50. What can be inferred about the water near the bottom of oceans?

　　(A) It is relatively warm.

　　(B) Its salinity is relatively high.

　　(C) It does not move.

　　(D) It evaporates quickly.