This is the factory---his father has worked for 20 years.空中填

答案：where
which 指代物,that指人指物,that 和which在定语从句中要作主语或者宾语,而这里很显然,不缺少主语或者宾语,所以用where,where 是关系副词,指地点,在句中作状语.