How would you explain the importance of ad impressions to a client who's concerned that her Search Network campaign is generating impressions but no clicks?

How would you explain the importance of ad impressions to a client who's concerned that her Search Network campaign is generating impressions but no clicks?
  A)  They can help her calculate how often someone clicked on her ad and then converted
  B)  They can help her evaluate how engaging her ad is to potential customers
  C)  They can give her an idea of how often her ad is shown to potential customers
  D)  They can give her an idea of how often someone clicked on her ad