有这样一个句子：
However, the best representatives for Beijing are the vanishing Hutongs and square courtyards. Through hundreds of years, they have become symbol of Beijing’s life. Tian’anmen square is still brilliant today with cloverleaf junctions and skyscrapers everywhere.
其中的 they have become symbol of Beijing’s life，为什么没有在 symbol 前用冠词？另外，symbol 为什么不用复数？因为句子主语 they 是复数啊！