They think of a plan by which they could fulfill their task ahead of time.上面这句的by which甚么意思?作甚么成分?

They think of a plan by which they could fulfill their task ahead of time.
上面这句的by which甚么意思?作甚么成分?

定语从句.“他们可以按时完成任务的”计划.

意思是by the plan
作方式状语，通过这个计划，他们可以提前完成任务

定语 通过这个计划