曼哈顿的一题！！

zoe爱猕猴桃

A powder derived from the North American Echinacea flower, which has been effective in preventing colds, is grown by many small farmers out West.

• A
  
  A powder derived from the North American Echinacea flower, which has been effective in preventing colds,
  
  
  
• B
  
  A derivative, which has been effective in preventing colds, of North American Echinea flower,
  
  
  
• C
  
  North American Echinacea flower derivative, which has been effective in preventing colds
  
  
  
• D
  
  The North American Echinacea flower has a derivative which has been effective in preventing colds, that
  
  
  
• E
  
  The North American Echinacea flower, a derivative of which has been effective in preventing colds,
  
  
  
  

求助！！答案选E，想问中间的of which怎么解释呀，of做derivative的介词，which指代花花么？？ 这也太跳跃了把。