单选题 ______ in the United States, St. Louis has now become the 24th largest city.
A. Being the fourth biggest city B. It was once the fourth biggest city
C. Once the fourth biggest city D. The fourth biggest city it was

【正确答案】 C

【答案解析】[解析] 根据句子结构，我们可以将B项和D项排除。A项中the fourth biggest city与主句中的the 24th biggest city相矛盾。正确答案为C项。Once在这里作连词，引导一个省略了主语和be形式的谓语。例如：Love, like Reputation, once fled, never returns. 爱情像名誉，一旦失去，永不再来。Love is like a mirror: once broken, that ends it. 爱情就像一面镜子：一旦破碎，断难重圆。