Scientists recently discovered that three different types of squid, a marine animal, previously thought to be three different species, were actually all members of one species. Their earlier ideas were based on using squid carcasses (dead bodies). The new, more accepted classification is most probably based on an analysis of
A. a greater number of squid carcasses
B. the feeding habits of the three different species
C. a number of newly found squid fossils
D. the DNA present in the cells of squid