A small cloud moves slowly through the air in a hospital operating room （手术室）.In the room a patient with cancer（癌症），lies on a table.The cloud is all around the patient，covering her body and filling her lungs（肺）.The cloud is not smoke.It is made of millions of nano﹣robots（纳米机器人）.These tiny robots move from cell （细胞） to cell in the patient's body，killing the cancer.

    Fighting cancer with nano﹣robots is only an idea today，but scientists say that it could be possible in the future.Using nano﹣robots for good purposes such as fighting the illness or repairing the environment may be the way to many of today's problems.

    However，nano﹣robots could be a dangerous risk as well.Because nano﹣robots are so tiny，they will have to work in large teams of many thousands to many millions.For this reason，nano﹣robots will have to be programmed（被编程）to build themselves.Scientists will not have the ability to build millions of nano﹣robots one by one.

    This ability to re﹣produce is making some scientists worried.What will happen if something goes wrong in the programming？Quite a lot of crazy nano﹣robots would be far worse than any illness.Some people think that if they get out of control，nano﹣robots could make the Earth gone.

    What should be done？Should we continue researching nano﹣robots or should we search for other methods？This is one of the many difficult decisions that we have to make in the future.

（1）What is the cloud in the passage？ 　 　

A.Smoke in the air.

B.Millions of nano﹣robots.

C.Something full of drops of water.

D.Clothes to cover a patient.

（2）What does the underlined word it in Paragraph 2 refer to（指代）？ 　 　

A.Fighting cancer with nano﹣robots.

B.Moving from cell to cell.

C.Killing the cancer.

D.Repairing the environment.

（3）Which of the following is NOT the reason why some scientists are worried about using nano﹣robots？ 　 　

A.Nano﹣robots have the ability to build themselves.

B.Something may go wrong in the programming.

C.Scientists have to build millions of nano﹣robots one by one.

D.Nano﹣robots in large teams may be out of control.

（4）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.

（5）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