凭什?;为何;因此
Whereby \Where*by"\, adv. 1. By which; -- used relatively. ``You take my life when you take the means whereby I life.'' --Shak. 2. By what; how; -- used interrogatively. Whereby shall I know this? --Luke i. 18.
whereby adv 1: as a result of which 2: by or through which; "the means whereby we achieved our goal"