10. What does the underlined word “it” in Paragraph 2 refer to?
A. Fighting cancer with nano - robots.
B. Moving from cell to cell.
C. Killing the cancer.
D. Repairing the environment.

11. What does the writer think of using nano - robots in the future?
A. He agrees to the idea.
B. He disagrees to the idea.
C. He doesn't talk about his own opinion.
D. He is crazy about it.

12. What is the best title?
A. Future Robots
B. The Ways to Fight Cancer
C. Nano - robots in Use
D. A Cloud of Hope and Danger

How do we know the time? A clock, a watch or a mobile phone can help us. However, many years ago there were no clocks and knowing the time was not so easy. Over the centuries people have developed different ways of telling the time.
About 5,500 years ago, the Egyptians invented the sun clock. This was a tall stone building. Its shadow (影子) showed the movement of the sun. So people were able to know midday. The Egyptians made a sundial (日晷) about 3,500 years ago. It was smaller than the sun clock and could let people know the time for half a day. On cloudy days or at night it was impossible to tell the time with a sun clock or a sundial. Water clocks were the first clocks not to use the sun. The idea is simple. Water flows (流动) from one bottle to another. When the water reaches a certain level, it shows the hours. The Egyptians used water clocks about 3,400 years ago. These clocks were popular in the Middle East and China. But they failed to make people know the exact time.
In the 13th century, the mechanical clock was invented. This was more exact, but it was expensive to make one. Over the next few centuries it was developed. For example, springs (发条) were added. This enabled people to get the exact time and allowed clocks to be