**Core Concept**: Synonym discrimination (subtle vs. obvious/direct/noticeable). **Analysis**: - **Correct Answer (A)**: 'Subtle' means 'not immediately obvious,' aligning with the context of influences that are hard to detect. This matches the original text's emphasis on unconscious imitation. - **Error Options**: - B. 'Obvious' contradicts the description of hidden influence. - C. 'Direct' implies explicit causation, inconsistent with implied indirectness. - D. 'Noticeable' conflicts with 'hard to notice immediately.' **Key Insight**: The sentence structure ("hard to notice") directly signals the need for a word meaning 'not easily perceived.' **Common Pitfall**: Mistaking 'noticeable' as neutral, but it still implies perceptibility.