辛亥革命前后，中国一直处在一种“国家危机”或“权威危机”之中，不论是改革派还是革命派一直都在努力探索一种新的国家形式。辛亥革命及民国的建立，意味着传统集权官僚体制崩溃。民国实质上是一个“军绅政权”，新的中央政府仍徒具形式，在“共和”的招牌下，国家实际陷于军阀割据和长期分裂状态。材料主要说明（  ）

魏晋时期，北方游牧民族逐渐涌入中原，农牧经济格局受到冲击，牧业开始向南推进；随着西晋的灭亡，游牧民族如潮水般涌入内地，畜牧带大幅度向南推移。由此推断魏晋时期(   )

B

    We all need to exercise. Doctors say it is good for us. It makes our heart and body strong. It also gives you more energy(能量). And you will feel better about yourself. It's best to exercise twice a week. Twenty minutes each time is enough.

    There are many ways to exercise. You can walk, run, play sports or swim. Many people go to special places to exercise. They are called "fitness centers". These places have a lot of equipment(设备). Some people buy equipment for their homes. But it's very expensive.

    Exercising can be fun. Friends can exercise together at fitness centers. Or they can play sports together. How do you exercise? How often do you exercise?