Martha Has Created Two Ads Through Google AdWords

Martha has created two ads through Google AdWords. Both ads show that they are eligible, but one ad has the message 'Bid is below the first page bid estimate' and a value for the bid. Which one of the following is an improvement Martha should try, to optimize her ads?

A) Change the destination URL of her ad
B) Change her ad to an image ad
C) Increase the ad's cost per click bid
D) Increase the ad's cost per impression bid