Polar bears inhabit the Arctic, on the outer perimeters of the sea ice. to be exact. When polar bear die, their bones almost always end up deep down on the floor of the icy Arctic Ocean. They basically disappear forever. Recently, however, a polar bear's jawbone was found within the Arctic Circle, on an island in Noway Svalbard archipelago. This bone held in a valuable information regarding the polar bear's revolutionary past. For instance, exactly how long the polar bear has existed as a species has been a matter of wild debate. So researchers took the opportunity to performdating analyses on the bone, and also to analyze its DNA. The bone was determined to be about 120,000 years old, and the DNA analysis established that it was indeed the bone of a polar bear. Now, in order to map out the polar bear's genetic profile, researchers performed mitochondrial DNA analysis. Usually when we think of DNA, 。。。。余下托福听力真题原文省略！
Question 1 of 6
What is the main purpose of the lecture?
O A. To compare the behaviors of three different bear species
O B. To examine changes to the habitat in which polar bears live
O C. To discuss recent insights into polar bear evolution
O D. To illustrate a new method of DNA analysis used in climate studies
Question 2 of 6
Why did the researchers that the professor mentions use mitochondrial DNA for testing instead of nuclear DNA?
O A. Mitochondrial DNAhas a more stable structure than nuclear DNAhas.
O B. Mitochondrial DNAhas a shape that is more easily identifiable than that of nuclear DNA.
O C. Mitochondrial DNAis present in larger quantities in cells than nuclear DNA.
O D. Mitochondrial DNAanalysis is a better-understood research tool than nuclear DNAanalysis