Researchers manipulated the tail length in male widowbirds and compared female preference for birds ...

Researchers manipulated the tail length in male widowbirds and compared female preference for birds with differing tail lengths. Do the results that are reported in the figure below show that male tail length is an hon-est signal of male quality?
 
  a. Yes, females would otherwise not prefer males with longer tails.
  b. Yes, the longer the tail the more energy males need to spend on growing it.
  c. No, just putting a longer tail on a male does not increase its quality.
  d. No, this study did not test whether birds with naturally long tails were of higher quality than birds with naturally short tails.