which的先行词不是一个词，而是指代 From April 2nd to the 4th,visitors whose surname is Zhu can get free tickets for several viewing areas of the Ming Dynasty Tomb这个句子。

    类似的例句：He got married again a year later, which surprised everybody. 他一年之后又结婚了，这使大家都感到意外。（摘自《牛津英语用法指南》第三版494.9节），这里的 which 指代 He got married again a year later 整个分句。