题目

China says 16 endangered finless porpoises (江豚) have been found dead since the beginning of the year and experts blame water pollution and climate change for pushing the species toward extinction (灭绝). The freshwater porpoise — popularly known in Chinese as the “river pig” — mainly lives in China’s Yangtze River and two lakes linked to the river. Authorities (官方) have discovered 10 dead porpoises in Dongting Lake in the central province of Hunan since March, according to a statement by the government of Yueyang City, which is located on the lake’s shore. Another 已知△ABC中，AB=AC=10，BD是AC边上的高线，DC=2，那么BD等于A.4B.6C.8D.