问题补充:
The book represents a wonderful love story, so once, it will be popular with young people.
A. publishes
B. publishing
C. published
D. having published
答案:
A. publishes
B. publishing
(答案→)C. published
D. having published
解析: once作为连接词引导条件状语从句, 后面可以接现在分词或过去分词,由于it与publish为被动关系, 所以用过去分词published作状语。