）Beijing is one of the in the world today.A. busiest cityB. busiest cities C. busy city D

问题补充：

）Beijing is one of the in the world today.

A. busiest city

B. busiest cities

C. busy city

D. busy cities

答案：

A. busiest city

(答案→)B. busiest cities

C. busy city

D. busy cities

解析：句意：今天北京是世界上最繁忙的城市之一。结合语境可知本句表示在多个事物中进行对比，故用形容词的最高级，指的是世界上的多个城市中，故city用复数形式，选B。