Hồ Baikal, Hồ lớn nhất trên thế giới, nằm ở phía Nam Siberia, Nga. Nó biện pháp 395miles of coastline. The lake’s rocky basin consists of three depressions, which hold a total volume of 14,656 cubic miles of water, 20 percent of the freshwater in the world. The lake is also very deep, with its deepest point at over 1 mile and an average depth of 2,066 feet. Furthermore, scientists have determined that Lake Baikal is the oldest lake in the world. From sediment obtained by drilling deep down below the lake, researchers estimate it to be at least 25 million years old. To scientists, Lake Baikal is of particular interest because of its unique and isolated ecosystem. More than 1,000 species of plants and animals found at Lake Baikal exist nowhere else on earth and some can be dated to prehistoric times. Among its unique fauna is the Baikal freshwater seal. This creature, which local people call nerpa, is the only mammal which inhabits the lake. Researchersspeculate that these seals, which have been breeding at Lake Baikal for 22 million years, are the descendents of ocean-dwelling seals which migrated inland in search of food, when the lake was still connected to the sea. Another creature that is unique to this Siberian Lake is the omu, a fish, which is caught by local fisherman and is considered a great delicacy. For centuries, the water in Lake Baikal was so clear that it was possible to see down to depths of 40 to 60 feet. First, the lake contained certain small zooplankton and small crustaceans that consumed waterweeds, bacteria, and other material that would otherwise cloud the water. Furthermore, the water in the lake consisted of rainwater and melted snow that flowed down from a mountain range in about 300 streams and through uninhabited forest. Finally, most of the watershed has a rocky surface, so the water flowing into the lake did not accumulate mud or organic matter and had little mineral or chemical content.Câu 71: The word “which” in the first paragraph refers to __ .A. the rocky basin B. the lakeC. the total water volume D. three depressionsCâu 72: All of the following is mentioned EXCEPT that __A. Lake Baikal is the world’s largest lake B. Lake Baikal is located in RussiaC. Lake Baikal is the deepest lake in the world D. Lake Baikal is the oldest lake in the worldCâu 73: All of the following are true about Lake Baikal EXCEPT that __A. it has a great variety of fishB. it is near the seaC. it has the biggest amount of freshwater in the worldD. it is about 25 million years oldCâu 74: Scientists are particularly interested in Lake Baikal because __A. its ecosystem is specialB. the animals there are similar to elsewhereC. it has more species of plants than anywhere elseD. it is isolatedCâu 75: Baikal fresh seal is __A. the only habitants of the lake BaikalB. only found in the lake BaikalC. has been breeding at the lake for 32 million yearsD. a species of plantCâu 76: Omu is mentioned in the passage as __.A. a local fisherman B. an animal migrating inland from the seaC. descendents of ocean-dwelling seals D. an animal unique to the lakeCâu 77: The water of the lake Baikal is __A. used to make crystal B. toxicC. made of crystal D. transparentCâu 78: According to the passage, there are __ factors contributing to the clarity of the water in Lake Baikal.A. 2 B. 1 C. 3 D. 4Câu 79: The word cloud in the final paragraph can be best replaced by __A. cold B. dirty C. clean D. muddyCâu 80: The water in the lake is from all of the following EXCEPT __A. snow B. rainwater C. streams D. forests
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