The number of private cars in urban areas should be limited the number of public buses should be increased.
A. when
B. while
C. unless
D. until
网友回答
A. when
(答案→)B. while
C. unless
D. until
解析：考查连词：A. when当…时候B. while 当…时候，然而C. unless除非D. until直到，句意：都市的私家车数量应该减少而公共汽车的数量应该增加。选B。