symbol前用不用冠词与用不用复数的疑问

问题描述

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！

老师答疑

黎老师：

symbol 是可数名词，单数形式应该与冠词连用，例如：

Mandela became a symbol of the anti-aparthied struggle. 曼德拉成为反种族隔离斗争的象征。（《牛津高阶英汉双解词典》第8版p.2117）

所以，我认为 they have become symbol of Beijing’s life，中的 symbol 应该改为复数形式。