题目

To tweet, or not to tweet? A guide to the social networking/ microblogging service Tewitter SINCE its creation in 2006, Twitter, the social networking service, has taken cyber space by storm. At first glance it might seem like Facebook, but Twitter is in a league of its own, connecting people with fast-paced updates. It has become a place for activities, celebrities, businesses and everyday people to let others know about videos, opinions, interesting news, advertisements-and, yes what they are eating for lunch. The phenomenon includes a host of new vocabulary terms and concepts that every sel 2008年9月25日至28日我国成功实施了“神舟”七号载人航天飞行并实现了航天员首次出舱。飞船先沿椭圆轨道飞行，后在远地点343千米处点火加速，由椭圆轨道变成高度为343千米的圆轨道，在此圆轨道上飞船运行周期约为90分钟。下列判断正确的是（      ） A．飞船变轨前后的机械能相等 B．飞船在圆轨道上时航天员出舱前后都处于失重状态 C．飞船在此圆轨道上运动的角速度大于同步卫星运动的角速度 D．飞船变轨前通过椭圆轨道远地点时的加速度大于变轨后沿圆      轨道运动的加速度