**Rule: Understanding Idiomatic Expressions.** The idiom 'hear through the grapevine' means to hear news or information through an informal, person-to-person chain of communication, like gossip or rumor. It implies the information is not from an official source. Option (d) is the correct definition. Options (a) and (b) describe formal communication, the opposite of the idiom. Option (c) is a literal and imaginative misinterpretation.

**Rule: Understanding Idiomatic Expressions.** The idiom 'all bark and no bite' refers to someone who is full of angry threats but is not willing or able to act on them. The sentence itself explains the meaning by contrasting his 'threatens and shouts' with the fact that he would 'never actually harm anyone.' Option (b) is the correct definition. Option (c) is the literal origin of the idiom but refers to an animal, not a person. Options (a) and (d) describe different personality types.

**Rule: Understanding Idiomatic Expressions.** The idiom 'in the dark' means to be in a state of ignorance, lacking information about something. In this case, the police lack information about the 'identity of the thief.' Option (b) correctly defines this state. Options (a) and (c) are literal interpretations of 'dark.' Option (d) is irrelevant.

**Rule: Understanding Idiomatic Expressions.** The idiom 'from scratch' means to start from the very beginning, without using anything that has been prepared before. A business failure often means losing everything and having to begin anew. Option (a) is the correct definition. Option (b) is a possible action but not the meaning of the idiom. Option (c) is the opposite. Option (d) is a literal and incorrect interpretation of 'scratch'.

**Rule: Understanding Idiomatic Expressions.** The proverb 'you can't judge a book by its cover' is a metaphor that means one should not prejudge the worth or value of something or someone by their outward appearance alone. The sentence uses this to caution against trusting someone just because they 'seem friendly.' Option (b) is the correct explanation. Option (a) and (c) are incorrect literal advice about books. Option (d) is the exact assumption the proverb warns against.

**Rule: Understanding Idiomatic Expressions.** The idiom 'call it a day' means to decide to stop doing something, especially working, for the rest of the day. The context of having worked for 'ten hours straight' and postponing the rest of the work makes the meaning clear. Option (b) is the correct definition. Option (a) is a literal misinterpretation. Options (c) and (d) are unrelated actions.

**Rule: Understanding Idiomatic Expressions.** A 'cock and bull story' is an improbable, elaborate, and false story or excuse. The subject of being 'abducted by aliens' is a classic example of something unbelievable. Option (c) is the correct definition. Option (a) is a literal interpretation. Option (b) is the opposite. Option (d) describes the length, not the credibility, of a story.

**Rule: Understanding Idiomatic Expressions.** The idiom 'hit the sack' (or 'hit the hay') is a colloquial expression that means to go to bed or go to sleep. The context of a 'long day of hiking' suggests tiredness and the need for rest. Option (c) is the correct meaning. Option (a) is a literal interpretation of 'hit the sack.' Options (b) and (d) are unrelated actions.

**Rule: Understanding Idiomatic Expressions.** The idiom 'the elephant in the room' refers to a significant and obvious problem or topic that everyone is aware of but avoids discussing. The phrase 'nobody wants to discuss' is a direct clue. Option (b) correctly defines this idiom. Option (a) is a literal and alarming misinterpretation. Option (c) is the opposite. Option (d) is a possible but incorrect literal interpretation.

**Rule: Understanding Idiomatic Expressions.** The idiom 'stick to one's guns' means to maintain one's position or opinion, especially in the face of opposition. The context 'even when everyone else disagrees' perfectly illustrates this meaning. Option (b) is the correct definition. Options (a), (c), and (d) are all incorrect, literal interpretations of the word 'guns'.