【正确答案】 D

【答案解析】[解析] 划线词subject意为：主题。A项question意为：问题；例句：Answer my questions without hesitation.要毫不犹豫地回答我的问题。B项problem意为：难题，习题；例句：A new problem has been reared up in production.生产中出现了一个新问题。C项title意为：标题，头衔；例句：The title of the novel is "Sons and Lovers".这本书的书名为《儿子与情人》。D项topic意为：主题，话题；例句：The topic has been studied by many investigators.这个课题已经被很多研究者探索过。故本题选D。