A、3,000m. B、3,000km. C、6,000m. D、6,000km. A

W: Today’s guest on "Science Update" is David Brown. [1]Dr. Brown, you and your team have found bacteria far below the Earth’s surface. You must be thrilled about your discovery.
M: Well, yes, it’s very exciting. For a long time we’d suspected the presence of such organisms, but we lacked substantial evidence.
W: How did you confirm the existence of the bacteria?
M: [2]Well technology helped. Our drilling techniques have improved significantly, and so the risk that surface bacteria could be mistaken for those found at much greater depth was reduced. With the new techniques, we could get much deeper into the Earth.
W: How far down did you actually get?
M: [3]In one case, about three kilometers. We were surprised, I must tell you, that there were organisms that far down.
W: You know, it sounds like fiction, something like a lost world.
M: Let’s call it a hidden biosphere, and it’s probably a very extensive one. The mass of the living organisms below the surface may be equal in size to the mass of the surface bacteria.
W: Have you found any unique life-forms?
M: Yes. We’ve found a very special organism. Let’s call it Type-A bacterium. It can live and grow only where there is no oxygen.
W: Is there any danger of these bacteria infecting people when you bring them to the surface?
M: These bacteria were adapted to an environment that’s completely different from humans’. [4]That’s to say, they could not survive in our environment. So we really don’t need to worry about these bacteria causing illness in people.
1. What have Dr. Brown and his team discovered?
2. What helps to confirm the existence of bacteria below the Earth’s surface?
3. How deep down did the man find the bacteria?
4. Why does the man say there is no danger of infection by the bacteria brought to surface?