Câu hỏi:

Part 2. Questions 66 – 75 (2.0 points - 0.2/ each)

Read the passage below and choose the best answer to each question

          Lake Baikal, the world’s largest lake, is located in southern Siberia, in Russia. It measures 395 miles long by 50 miles wide and it has 1,245 miles 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. Researchers speculate 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 LakeBaikal was so clear that it was possible to see down to depths of 40 to 60 feet. Several factors contributed to keeping the water crystal clear. First, the lake contained certain small zoo plankton 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.

The word “which” in the first paragraph refers to ________.