A、As a water-based paste. B、As a liquid. C、In stone-size blocks. D、In powdered form. D从“When this substance was ground into a fi

One of the primary materials used in the construction of buildings and roads is cement, a powder made primarily from limestone and clay. Even though the ancient Egyptians and Romans used a kind of cement, it was not until 1824 that an English bricklayer developed a cement strong enough for modern roads and buildings.
    While experimenting in his kitchen, the bricklayer found that a mixture of limestone and clay that had been heated together formed a hard, stone-like chunk as it cooled. When this substance was ground into a fine powder it could be stored indefinitely.
    When the powder was mixed with water, it made an excellent quality of cement that quickly hardened in sunlight or even under water. With only a few variations, this is how cement is made today.
Questions 26-30 are based on the passage you have just heard.
26. What is the main topic of the passage?
27. Who developed the kind of cement that is used today?
28. Where was modern cement first made?
29. How is cement stored?
30. According to the speaker, how does modern cement compare with that of 150 years ago?