Bird guides once listed the myrtle warbler and Audubons warbler as distinct species

QUESTION:

Bird guides once listed the myrtle warbler and Audubons warbler as distinct species. Recently, these birds have been classified as eastern and western forms of a single species, the yellow-rumped warbler. Which of the following pieces of evidence, if true, would be cause for this reclassification? a. The two forms interbreed often in nature, and their offspring survive and reproduce well. b. The two forms live in similar habitats and have similar food requirements. c. The two forms have many genes in common. d. The two forms are very similar in appearance.