Volcanic fire and glacial ice are natural enemies. Eruptions at glaciated volcanoes typically destroy ice fields， as they did in 1980 when 70 percent of Mount Saint Helens ice cover was demolished. During long dormant intervals， glaciers gain the upper hand cutting deeply into volcanic cones and eventually reducing them to rubble. Only rarely do these competing forces of heat and cold operate in perfect balance to create a phenomenon such as the steam caves at Mount Rainier National Park.

　　Located inside Rainier's two ice-filled summit craters，these caves form a labyrinth of tunnels and vaulted chambers about one and one-half miles in total length. Their creation depends on an unusual combination of factors that nature almost never brings together in one place. The cave-making recipe calls for a steady emission of volcanic gas and heat， a heavy annual snowfall at an elevation high enough to keep it from melting during the summer， and a bowl-shaped crater to hold the snow.

　　Snow accumulating yearly in Rainier's summit craters is compacted and compressed into a dense form of ice called firn，a substance midway between ordinary ice and the denser crystalline ice that makes up glaciers. Heat rising from numerous openings （called fumaroles） along the inner crater walls melts out chambers between the rocky walls and the overlying ice pack. Circulating currents of warm air then melt additional opening in the firn ice， eventually connecting the individual chambers and， in the larger of Rainier's two craters， forming a continuous passageway that extends two- thirds of the way around the crater's interior.

　　To maintain the cave system， the elements of fire under ice must remain in equilibrium. Enough snow must fill the crater each year to replace that melted from below. If too much volcanic heat is discharged， the crater's ice pack will melt away entirely and the caves will vanish along with the snow of yesteryear. If too little heat is produced， the ice，replenished annually by winter snowstorms， will expand， pushing against the enclosing crater walls and smothering the present caverns in solid firn ice.

　　45.With what topic is the passage primarily concerned？

　　（A） The importance of snowfall for Mount Rainier.

　　（B） The steam caves of Mount Rainier.

　　（C） how ice covers are destroyed.

　　（D） The eruption of Mount Saint Helens in 1980.

　　46.The word "they" in line 2 refers to

　　（A） fields

　　（B） intervals

　　（C） eruptions

　　（D） enemies

　　47.According to the passage long periods of volcanic inactivity can lead to a volcanic cone's

　　（A） strongest eruption

　　（B） sudden growth

　　（C） destruction

　　（D） unpredictability

　　48.The second paragraph mentions all of the following as necessary elements in the creation of steam caves EXCEPT

　　（A） a glacier

　　（B） a crater

　　（C） heat

　　（D） snow

　　49.According to the passage， heat from Mount Rainier's summit craters rises from

　　（A） crystalline ice

　　（B） firns

　　（C） chambers

　　（D） fumaroles

　　50.In line 37 "smothering" the caverns means that they would be

　　（A） eliminated

　　（B） enlarged

　　（C） prevented

　　（D） hollowed