centring of a lens

例句：

1. 1. The city is the center of ourcountry's economy.
   这个城市是我们国家的经济中心。
2. 2. The heart of the matter liesin understanding the underlying causes.
   问题的核心在于理解根本原因。
3. 3. The company's headquartersare located at the center of the city.
   公司的总部位于市中心。