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HOST: What's the capital city of South Africa? is it Cape Town, Pretoria or Johannesburg?
PAUL: Hm, that's difficult. What do you think, John?
JOHN: I don't know. It (1) could / must be Cape Town.
DIANA: No, it (2) can't / must be Cape Town: because that's on the coast and I'm sure the capital city isn't on the coast.
PAUL: Yes, you're right. So it (3) can't / could be Pretoria or Johannesburg.
JOHN: Can we have a clue?
HOST: Yes, okay. It isn't Johannesburg.
PAUL: Then it (4) could / must be Pretoria.
HOST: That's right. Here's your next question. Which is the longest river in Europe? Is it the Volga, the Nile, the Danube or the Rhine.
PAUL: Well, it (5) could / can't be the Nile because that's in Africa.
JOHN: And I know the Rhine is shorter than the Danube, so it (6) can't / must be that.
PAUL: I don't think it's the Volga. Is that in Europe?
DIANA: Yes, it is. So it (7) could / can't be the Danube or the Volga.
PAUL: Can we ask for another clue?
HOST: Yes, the river doesn't go through Hungary.
DIANA: Then it (8) can't / must be the Volga because the Danube goes through Budapest, which is the capital of Hungary.