A、They defend the bone against viruses. B、They prevent oxygen from entering the bone. C、They break down bone tissue. D、They conn

W: ok, last night you were supposed to read an article about human bones.  Are there any comments about it?  
M: well, to begin with, I was surprised to find out there were so much going on in bones. I always assumed they were pretty lifeless.  
W: Well, that’s an assumption many people make. But the fact is bones are made of dynamic living tissue that requires continuous maintenance and repair.  
M: Right. That’s one of the things I found so fascinating about the article the way the bones repair themselves.  
W: Ok. So can you tell us how the bones repair themselves.  
M: Sure. See, there are two groups of different types of specialized cells in the bone that work together to do it. The first group goes to an area of the bone that needs repair. This group of cells produce the chemical that actually breaks down the bone tissue, and leaves a hole in it. After that the second group of specialized cells comes and produce the new tissue that fills in the hole that was made by the first group.  
W: Very good. This is a very complex process. In fact, the scientists who study human bones don’t completely understand it yet. They are still trying to find out how it all actually works. Specifically, because sometimes after the first group of cells leaves a hole in the bone tissue, for some reason, the second group doesn’t completely fill in the hole. And this can cause real problems. It can actually lead to a disease in which the bone becomes weak and is easily broken.  
M: ok, I get it. So if the scientists can figure out what makes the specialized cells work, maybe they can find a way to make sure the second group of cells completely fills the hole in the bone tissue every time. That’ll prevent the disease from every occurring.