摘要：a用于抽象名词之前表示具体介绍. “a+抽象名词 起具体化的作用, 表示 “一个什么样的人/事/物 a. This little boy is a joy to his parents. b. This book is a great help to me. c. The party is a great success. d. It is a pleasure to talk with you.

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