下列关于地球同步卫星的说法中正确的是（　　）

A．为避免通讯卫星在轨道上相撞，应使它们运行在不同的轨道上

B．通讯卫星定点在地球赤道上空某处，所有通讯卫星的周期都是24h

C．不同国家发射通讯卫星的地点不同，这些卫星的轨道不一定在同一平面上

D．不同通讯卫星运行的线速度大小是相同的，向心力的大小也是相同的

Many of the activities performed during the preliminary investigation are still being conducted in(1)，but in much greater depth than before.During this phase,the analyst must become fully aware of the(2)and must develop enough knowledge about the(3)and the existing systems to enable an effective solution to be proposed and implementeD. Besidesthe(4)for process and data of current system,the deliverable from this phase also includes the(5)for the proposed system.

空白（2）为（）

A.main symptom

B.root problem

C.final blueprint

D.data specification