均匀的上半椭球体, (, , )的重心为
A、
B、
C、
D、

单项选择题下图要完成10进制计数器，数据端数据D3D2D1D0=（ ）。

单项选择题Part 2: Multiple Choice. (20%)Directions: Choose the right choice to complete the sentences. 21. In previous times when fresh meat was in short supply, pigeons were kept by many households as a(n) ____ of food. A、origin B、resource C、means D、source