in the United States, St Louis has now bee the 4th largest city.
A. It is the 24th biggest city
B. It was the 24th biggest city
C. Before the 24th biggest city
D. Once the 24th biggest city
网友回答
A. It is the 24th biggest city
B. It was the 24th biggest city
C. Before the 24th biggest city
(答案→)D. Once the 24th biggest city
解析：句意：那是St Louis是美国排名第24的大城市，而如今它已成为排名第4的大城市。句子中间用逗号隔开，说明整个就是一个句子，那就不能存在两个谓语，除非其中一个是从句，所以去掉A、B两个答案，再根据句意，故选D。