使用光学显微镜时，观察的材料必须是（  ）

    The Galapagos tortoise (加拉巴哥象龟) is a kind of tortoise. In Spanish, "Galapagos" is a word for "tortoises". The Galapagos tortoises gave their name to the Galapagos Islands because they were first found there.

    Do you know what Galapagos tortoises look like? They can grow to be five feet long and weigh over one hundred and fifty kilograms. An adult tortoise is strong enough to lift a man.

    Galapagos tortoises can walk for a long time without food. They can survive on very little water for a few weeks. The female tortoises lay eggs in a hole in the sand or among the rocks. The babies are very small with soft shells. The babies always face dangers from other animals. There are many animals which eat the eggs and the baby tortoises. In fact, only one in about three thousand survives to become an adult tortoise. Adults often live for more than one hundred years.

    Now Galapagos tortoises are endangered because people eat the eggs they lay and the adult tortoises are hunted for their big shells. Also, the pollution of the Galapagos Islands is hurting the Galapagos tortoises as well. The number of them is becoming smaller and smaller. We should start to protect them.

下列关于我国高原地形的叙述，指黄土高原的是(     )

能源生产结构是指某类能源的生产量占能源生产总量的比例。能源消费结构是指某类能源的消费量占能源消费总量的比例。读下图我国煤炭和石油生产与消费结构差，回答下列各题。