a built-up city的用法和样例：
- An age builds up cities; an hour destroys them.
- The speed of the car was reduced from 40 mph to 30 mph when entering a built up area.
- He has built up a good business over the years.
- One team has already built up a commanding lead.
- Acne is usually caused by a combination of factors including a build up of dead skin cells, excessive sebum production, and bacterial overgrowth.