Câu hỏi:

Read the following passage, and choose the correct answer to each of the questions     

       A rather surprising geographical feature of Antarctica is that a huge freshwater lake ,one of the world’s largest and deepest , lies hidden there under four kilometers of ice .Now known as Lake Vostok, this huge body of water is located under the ice block that comprises Antarctica . The lake is able to exist in its unfrozen state beneath this blockof ice because its waters are warmed by geothermal heat from the earth’s core .The thick glacier above Lake Vostok actually insulates it from the frigid temperatures on the surface .      The lake was first discovered in the 1970s while a research teamwas conducting an aerial survey of the area .Radio waves from the survey equipment penetrated the ice and revealed a body of water of indeterminate size.It was not until much more recently that data collected by satellite made scientists aware of the tremendous size of the lake ; the satellite –borne radar detected an extremely flat region where the ice remains level because it is floating on the water of the lake .      The discovery of such a huge freshwater lake trapped under Antarctica is of interest to the scientific community because of the potential that the lake contains ancient microbes that have survived for thousands upon thousands of years, unaffected by factors such as nuclear fallout and elevated ultraviolet light that have affected organism in more exposed areas .The downside of the discovery,however, lies in the difficulty of conducting research on the lake in such a harsh climate and in the problems associated with obtaining uncontaminated samples from the lake without actually exposing the lake to contamination . Scientists are looking for possible way to accomplish this. 

The word “hidden” in paragraph 1 is closest in meaning to