Core concept: Synonyms辨析。 解题思路: 'Decline' in formal contexts often refers to gradual reduction. 选项分析: A. 'diminish' implies gradual decrease (correct). B. 'refuse' means reject (incorrect verb). C. 'descend' refers to physical movement (context mismatch). D. 'reduce' involves active cutting (different agency). 易错点: Confusing physical descent (C) with quantitative reduction.