4. Let these two points be A and B, then there is a ray on the left and a ray on the right of point A, and so is point B, so * * * has four rays.

There are four rays. If two points on a straight line are A and B, A is the endpoint, a ray on the left, a ray on the right, and B is the endpoint, the same holds true.

This kind of problem is very simple. You only need to see how many vertices there are, one vertex is two rays (it doesn't matter how many vertices there are, because the rays are infinitely long on one side), and those two vertices are four.

introduce

Because of the different ionization density, the biological effects of various rays are different, and the change rate is also different. In order to obtain higher favorable mutation, it is necessary to choose appropriate rays, but due to factors such as ray source, equipment conditions and safety, gamma rays and X rays are the most commonly used at present.

Visible light, infrared light, ultraviolet light, etc. Is caused by electrons from the outer layer. Roentgen rays are caused by internal electrons. Gamma rays are caused by nuclei.