Puzzle 10: Truth and Liar among 3 friends

Puzzle 10: There are 3 friends say A, B, C. Of them, C knows one of A/B speaks always the truth and the other speaks always lies. One day, all of them meet with a car accident. Unfortunately, both A and B die on the spot, and C joins them a bit late. When C reaches the alien world, he wants to goto heaven. He only sees 2 exits. So, he assumes one exit for heaven and the other exit for hell. He also recognizes both A and B as security guards standing at each of the exits. Now, knowing one of A/B speaks truth and the other lies always, C needs to find way to heaven, by asking only ONE question to each of the security at the exits? How?
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Solution:
a) Ask A, did he die? if he says NO, he’s the one who always lies. Ask B, which is the door to heaven and take that door.
b) If A says Yes, take the other door that B did NOT say went to heaven.