Three spies, suspected as double agents, speak as follows when questioned: Albert: "Bertie is a mole." Bertie: "Cedric is a mole." Cedric: "Bertie is lying." Assuming that moles lie, other agents tell the truth, and there is just one mole among the three, who is the mole?

“Albert”

If Albert were the mole, then his statement about Bertie being a mole would be a lie, which would imply that Bertie is not the mole. However, if Bertie is not the mole, then Cedric's statement about Bertie lying would also have to be a lie, leading to a contradiction since only one mole exists.

“Cedric”

If Cedric were the mole, then his statement that Bertie is lying would be a lie, indicating that Bertie is actually telling the truth. However, that would mean Bertie is not the mole, which contradicts the assumption that only one mole is present.

“All of them are moles”

The riddle specifies that there is only one mole among the three spies. If all were moles, it would contradict the premise of the riddle, as it requires only one to be lying.