A is the one who tells the truth. Let's start with the answer to D: 1. Because some of these four people must be telling the truth. Otherwise, no one will say "all four of us are lying". So d must be a liar. B is lying. If what B says is true, then B, C and A should all tell the truth, so that the answers of B and C should be consistent, but the answers of B and C are indeed contradictory, then B is a liar. 3. If C is lying, 1 means A is telling the truth; If C is telling the truth, it can be inferred from C's answer that A is telling the truth. So whether C is lying or not, it can be inferred that A is a truthful person.