A、Because it cannot be noticed by human senses. B、Because sometimes it can be detected by human sense. C、Because it may be all a

Nuclear radiation has a certain mystery about it, partly because it cannot be detected by human senses. It can’t be seen or heard, or touched, even though it may be all around us. There are other things like that. For example, radio waves are all around us but we cannot detect them, sense them, without a radio receiver. Similarly, we can’ t sense radioactivity without a radiation detector. But unlike common radio waves, nuclear radiation is harmful to human beings and other living tilings.
    At very high levels, radiation can kill an animal or human being outright by killing masses of cells in vital organs. But even the lowest levels can do serious damage. There is no level of radiation that is completely safe. If the radiation does not hit anything important, the damage may not be significant. This is the case when only a few cells are hit, and if they are killed outright, your body will replace the dead cells with healthy ones. But if the few cells are only damaged, and if they reproduce themselves, you may be in trouble. They reproduce themselves in a deformed way. They can grow into cancer. Sometimes this does not show up for many years.
    This is another reason for some of the mystery about nuclear radiation. Serious damage can be done without the victim being aware at the time that damage has occurred. A person can be irradiated and feel fine, then die of cancer five or twenty years later as a result. Or a child can be born weak or liable to serious illness as a result of radiation absorbed by its grandparents.
    Radiation can hurt us. We must know the truth.
Questions 22 to 25 are based on the passage you have just heard.
22. Why is nuclear radiation mysterious?
23. How can high levels of radiation kill an animal or human being?
24. Which of the following cases is the most dangerous?
25. What will happen if a person is irradiated?