With an appropriate relative pronoun or relative adverb:
This custom, 1 originated many centuries ago, is slowly disappearing.
  • 1、
【正确答案】 1、which    
【答案解析】 本句主语为this custom，谓语部分为is slowly disappearing，所以可以知道this custom后为非限制性定语从句，所以空格处填which。