Our class recently has had a heated discussion about if the historic old buildings should be tear down. Opinions varies from one to another. Some students support the protection of those buildings so they are carriers of history through which the future generations can deep understand the development of our city. Beside, the historic old buildings can also arouse people’s love for the city.

In the contrary, some other students think that the old buildings may hold back the development of economy and the process of modernization. It may affect the beauty of the city.

As far I’m concerned, I approve of（赞成） the former opinion. But if a building is worth of little, it may be proper to demolish(摧毁) it and make better use of that area. We should just weight the advantages and disadvantages.

2012年至2015年进入了我国北斗系统卫星发射的高峰期，北斗卫星系统由地球同步轨道卫星与低轨道卫星两种卫星组成，在正常轨道运行的这两种卫星比较 （　　）

A．低轨卫星运行的周期较大

B．同步卫星运行的周期较大

C．低轨卫星运行的加速度较大

D．同步卫星运行的线速度较大