A、Because people expose too much to the computer. B、Because people don’t care about the low levels of radiation. C、Because the c

Nuclear reaction is no longer an alien concept for us. It can not be detected by human senses. We cannot see or touch it. Radiation is similar with radio waves. Radio waves can not be sensed without a radio receiver. Radioactivity can not be sensed without a radiation detector. On the other side, there is a big difference between radio waves and radiation. Radiation is harmful to people’s health and other living things. Radiation at high levels can even kill living things by killing masses of cells in important organs before people know the damage in their bodies. No level of radiation is completely safe. If the cells are badly hit by radiations of low levels, die dead cells will be replaced by the healthy cells in your body. But if the cells are killed by high levels of radiation, the cells may produce themselves in an unusually way, and these cells may become cancerous. This is the origin of our increased risk in cancer, as a result of radiation exposure.
    We have studied radiation for nearly 100 years now. We know radiation does lots of damage. The hurt can be passed on from generation to generation. A child can be born very weak or easy to get serious illness as a result of radiation absorbed by its grandparents. Nowadays, the scientists have been exploring new ways to deal with the radiation. They are trying to find out ways to prevent the damage of radiation to human being’s health. To thoroughly solve the problem of radiation, more effort still needs to be taken.
Questions:
17. Why does the speaker say that radiation is similar to radio waves?
18. According to the passage, why does the cancer increase?
19. Which of the following is true according to the passage?
20. Which of the following can be inferred from the passage?