No longer do many cities in United States have a river as the focal point of urban life, but in San Antonio the river winds through the middle of the business district, and the River Walk, or Pasco del Rio, is the city’s most popular attraction. 我是在por宝典上看到这个题目的,改后是正确答案,可是我不太明白为什么no longer后边需要加上do?