Nuclear radiation （核辐射）has a certain mystery, partly because it cannot be detected （探测）by human senses. It can’t be seen or heard, or touched or tasted, even though it may be all around us. There are other things like that. For example, radio waves are all around us but we can’t detect them or sense them without a radio receiver. Similarly, we can’t sense radioactivity without a radiation detector. But unlike common radio waves, nuclear radiation is not harmless to human beings and other living things.
At very high levels, radiation can kill an animal or human being at once by killing plenty of cells in important organs(器官). 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 serious. This is the case when only a few cells are hit, and when they are killed at once. 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 an unusual 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 knowledge of the person at the time that damage has occurred. A person can be irradiated(放射治疗) and feel fine, then die of cancer five, ten, or twenty years later as a result. A child can be born weak or easy to get serious illness as a result of radiation absorbed by its grandparents.
According to the passage, the danger of nuclear power lies in       .

A．nuclear mystery	B．radiation detection
C．radiation level	D．nuclear radiation

Radiation can lead to serious trouble even at the lowest level        .   

A．when it kills a few cells

B．because the killed cells can’t be replaced

C．if it damages a few cells

D．unless the damaged cells can reproduce themselves

What does the author want to tell us?

A．Radiation can hurt us.	B．Radiation is a subject scientists work on.
C．Radiation is a mystery.	D．Radiation is the source of cancer.