English

The correct preposition to use with 'indifferent' in this context is 'to'. The phrase 'indifferent to his words' means he does not care about or is unaffected by his words.

The phrase "bird brain" is an idiomatic expression used to describe someone who is not very intelligent or acts foolishly, which matches option A.

The phrase "Achilles' heel" refers to a fatal weakness despite overall strength, originating from Greek mythology where Achilles was invulnerable except for his heel.

The sentence is a negative question tag construction. Since the main clause is positive ('It will rain soon'), the question tag must be negative ('won't it?'). This makes option B correct.

A person who walks on a tightrope or rope is called a funambulist. This term specifically refers to tightrope walkers, making option C the correct answer.

The correct answer is B because the sentence logically warns that unless he gives up smoking, he will not recover, which fits the context of a doctor's warning about health. The other options are grammatically incorrect or do not fit the warning context.

The idiom "spill the beans" means to reveal a secret or disclose information that was meant to be kept confidential. Option B correctly defines this idiom.

The phrase "to cry wolf" means to give a false alarm or raise a false warning, which matches option B. This idiom originates from the fable where a boy repeatedly falsely claims a wolf is attacking, causing others to ignore him when the wolf actually appears.

The phrase "when pigs fly" is an idiomatic expression used to indicate that something is impossible or will never happen. Therefore, option C is correct.

The phrase "best of both worlds" means having the advantage of two different opportunities or situations at the same time, which matches option C.