题干

欧盟和中国联合开发的伽利 略项目建立起了伽利略系统(全球卫星导航定位系统).伽利略系统由27颗运行卫星和3颗预备卫星组成,可以覆盖全球,现已投入使用.卫星的导航高度为2.4×104km,倾角为56°,分布在3个轨道上,每个轨道面部署9颗工作卫星和1颗在轨预备卫星,当某颗工作卫星出现故障时可及时顶替工作.若某颗预备卫星处在略低于工作卫星的轨道上,以下说法中正确的是(   )

A:预备卫星的周期大于工作卫星的周期,速度大于工作卫星的速度,向心加速度大于工作卫星的向心加速度

B:工作卫星的周期小于同步卫星的周期,速度大于同步卫星的速度,向心加速度大于同步卫星的向心加速度

C:为了使该颗预备卫星进入工作卫星的轨道,应考虑启动火箭发动机向前喷气,通过反冲作用从较低轨道上使卫星加速

D:三个轨道平面只有一个过地心,另外两个轨道平面分别只在北半球和南半球

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答案(点此获取答案解析)

B

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完形填空

    Imagine loving someone so much that you'd swim more than 5, 000 miles to see him or her. Dindim manages to swim that far every year to 1 his friend Joao. This is 2 not only because of the love between the two friends, but because Dindim is3a penguin. Joao found Dindim covered in oil, barely 4on a beach near his home. He picked him up, cleaned him off and brought him to his house where he 5 him and got him healthy again. Joao named him Dindim.

    After a week, he 6 the penguin back to the beach and tried to release him,7 Dindim refused to leave Joao. They 8 together for the next eleven months. Then, Dindim disappeared.9 Joao thought he'd never see his friend again. But just a few months 10 Dindim was back. He found Joao on the beach, and followed him home. Each year he 11four months to live with other penguins. For the other eight months, he lives with Joao. Joao says that each time they meet again, Dindim seems 12 to see him. He says he loves the penguin as if he were his own 13 He feels certain that Dindim loves him in the14way. Ecologist Carl Safina says that animals can and do love humans. He writes and speaks a lot about animals and their 15. According to Carl, it's obvious that animals feel love for humans. What's less obvious is whether or not humans love animals enough to 16 them. Hundreds of thousands of animals are 17 by the same oil that covered Dindim when Joao found him.

    Fifty percent of 18 has disappeared in the last 40 years because of humans'19 their habitats. Scientists say we're in a new mass extinction period. In the next two 20 they predict that we'll lose 75 percent of the remaining species on earth.