题目内容
听力原文： Many egglaying animals merely lay their eggs and leave. With turtles for instance, the eggs hatch on their own. The current theory about birds is that the earliest birds did just that, when they were coldblooded creatures living in warm places. However when they became warmblooded creatures living in cold places they had to remain on top of the eggs to keep them warm. This process we call incubation. For this they needed a place, a nest very likely the first nests were just primitive depressions scraped into the ground. Even now many species still lay eggs in this sort of crude nest. In fact, every spring a mother killdeer lays her eggs in scene pebbles along the edged the parking lot just outside this building. Primitive nests on the ground were fine for some birds but others began to elevate their nests onto Branches, perhaps to avoid predators. These early elevated nests were probably loose platforms of sticks and twigs, the types still built by ospreys and mostarians today. The latest evolutionary development in nests the most re cent version, so to speak, is the cupshaped nest. This is the one we regard today as the typical bird's nest, you know, like a robin's nest.
(33)
查看答案
搜索结果不匹配？点我反馈
更多问题
The U.S. Food and Drug Administration has acted against similar contamination incidents like that involving SV40 out of fear of a wider spread of AIDS.
______
______
听力原文： Most people think of astronomers as people who spend their time in cold observatories peering through telescope every night. In fact, a typical, astronomer spends most of his or her time analyzing data and may only be at the telescope a few weeks of the year. Some astronomers work on purely theoretical problems and never use a telescope at all. You night not know how rarely images are viewed directly through telescopes. The most common way to observe the skies is to photograph them. The process is very simple. First, a photographic plate is coated with a lightsensitive material. The plate is positioned so that the image received by the telescope is recorded on it. Then, the image cam he developed, enlarged and published so that many people can study it. Because most astronomical objects are very remote, the light we receive from them is rather feeble. But by using a telescope as a camera, long time exposures can be made. In this way objects can be photographed that are a hundred times tm faint to be seen by just looking through a telescope.
(20)
微信支付 
支付宝支付