I'm preparing the final exam for my master's degree, this is a problem of the past exam, it's confused me, don't know where to start.

My thought is the admissible heuristic is resolution rule, and then prove the resolution rule is admissible, is that right? if so, to prove the resolution rule is admissible, where should I start? thanks for helping guys.

Consider a theorem prover application. The A* algorithm can be used to search for the simplest (shortest) proof. Assume that the known axioms and theorems are represented as a knowledge base of Horn clauses in propositional logic and that the prover uses Backward Chaining.

(a) Propose an admissible heuristic.

(b) Prove that the proposed heuristic is admissible