Given the question: Read the following article and select the best answer. Article: Two years ago, the Funk family of suburban Chicago adopted a Chinese baby girl who had been abandoned on a sidewalk near a Yangzhou textile factory. Last year and halfway across the United States, the Ramirez family of suburban Miami adopted a girl who had been abandoned a week later on the same spot. Both families named their daughters Mia. It turns out, a first name and Chinese heritage aren't the only things the three-year-olds have in common. The girls' mothers--Holly Funk and Diana Ramirez--met on a website for parents who had gone through international adoptions. After a flurry of e-mails comparing photographs and biographical details, DNA testing proved the families' suspicions: The girls are probably fraternal twins. "I was in shock," said Ramirez, who lives with her husband Carlos in Pembroke Pines, Florida. "Well, now this is for real." The Internet and Web groups revolving around international orphanages are increasingly being used to link adopted children with biological kin . The site that the Funks and Ramirezes used has a membership of 137 people, with 15 sets of twins and seven sets of siblings whose relationships have been confirmed. At a reunion on Friday at Chicago's O'Hare International Airport, Mia Diamond Funk Mia Hanying Ramirez shyly surveyed each other, then reached for each other's hand. DNA tests established an 85 percent probability that the girls are at least half sisters. Scientists did not have a biological parent to test and reach a greater certainty, but given their ages and physical similarities, experts say it is likely they are fraternal twins. Douglas and Holly Funk hope to take Mia to Miami in October. Both sets of parents say they are committed to staying in touch and often let the twins talk to each other on the phone. Question: Experts are still not 100 percent sure that the two girls are fraternal twins because _ . - DNA tests are still not accurate enough - the two girls were born by different parents - the DNA of a biological parent is still missing - one girl is born a week later than the other
The answer is:
the DNA of a biological parent is still missing