80. 

This case is more interesting than the previous ones. There are four defendants: A, B, C, D. The following has been established:

1) If A and B are both guilty, then C was an accomplice.
2) If A is guilty, then at least one of the accused B, C was an accomplice.
3) If C is guilty, then D was an accomplice.
4) If A is not guilty, then D is guilty.

Who among the four defendants is undoubtedly guilty, and whose guilt remains in doubt?
80. First, let's prove that if A is guilty, then C is guilty. Suppose A is guilty. Then, by statement (2), either B or C is guilty. If B is not guilty, then C must be guilty. But suppose B is guilty. Then both A and B are guilty. Therefore, by statement (1), C is also guilty. This proves that if A is guilty, then C is guilty. Moreover, by statement (3), if C is guilty, then D is guilty. Combining these two facts, we conclude that if A is guilty, then D is guilty. But by statement (4), if A is not guilty, then D is guilty. Therefore, regardless of whether A is guilty or not, defendant D must be guilty. Thus, the guilt of D is beyond doubt. The guilt of all other defendants remains in doubt.