Who is the liar among A,B,C?

Question

Of three persons A, B and C, one always lies while the other two always speak the truth.

• C asked A: "Do you always speak the truth, yes or no?"
• A said something that C could not understand.
• So C asked B: "What did A just say?"
• B replied: "A said NO."

So, who is the liar?

Answer 1

The answer is:

B is the liar.

The reason:

If A is a truth-teller, he has correctly answered YES (I am a truth-teller).
If A is a liar, he also has answered YES (lying, as he is a liar).

So A must have said YES, and B must be lying when he claims that A said NO.