Which of the following statements are true?

(A) A sentence $\alpha$ entails another sentence $\beta$ if $\beta$ is true in few worlds where $\alpha$ is true.

(B) Forward chaining and backward chaining are very natural reasoning algorithms for knowledge bases in Horn form.

(C) Sound inference algorithms derive all sentences that are entailed.

(D) Propositional logic does not scale to environments of unbounded size.

Choose the correct answer from the options given below:

Statement (A) is false.

Entailment means if $\alpha$ is true, then $\beta$ must be true in all models where $\alpha$ is true.

Correct definition is:

$\alpha\models\beta$

means $\beta$ is true in every model in which $\alpha$ is true.

But statement (A) says few worlds, so it is false.

Statement (B) is true.

Forward chaining and backward chaining are natural reasoning algorithms for Horn form knowledge bases.

Statement (C) is false.

Sound inference algorithm derives only those sentences that are logically entailed.

But deriving all entailed sentences is the property of completeness, not soundness.

Statement (D) is true.

Propositional logic does not scale well to environments of unbounded size because it cannot represent general rules using variables and quantifiers.

Therefore, true statements are (B) and (D).