【正确答案】topic (reading) In the lecture, it refers to a key point that some aquatic organisms vertically migrate through bodies of water in order to find food or escape from predators. The professor illustrates this idea by showing the example of squids. example Based on the lecture, squids live on small creatures in the oceans. When there is strong sunshine, they can easily find the. food on water surface; however, they also attract predators under the sunshine and it will set themselves in danger. Therefore, squids travel to the bottom of the water and escape from their natural enemies in daytime, while migrating to the surface for food in the night. Under the circumstances, they don't need to be afraid of the attack from predators. In a word, squids keep moving between surface and bottom of the oceans everyday.

【答案解析】[听力原文] W: So, vertical migration. What is it exactly? Well, let's look at some organisms like squids. Squids feed on small fishes or tiny creatures. It's easy for them to find food on water surface in the daytime, because the sunlight is strong enough to help them. However, the same reason making their food easy to spot might also expose squids to their predators. Because of tills, squids usually swim to the bottom of the oceans or lakes during the day, waiting for the evening to come. Once the sun is set, squids can spend all night hunting for food with little or no distractions from predators under the curtain of night, because the eye sight of their enemies is relatively poor at night. When the night is over, squids return to the depths of water.