II. Text Completion 
Select the best answer to complete the following texts.
      "Every drop of human blood contains a history book written in the language of our genes," says
population geneticist and National Geographic Explorer-i -in-Residence, Spencer Wells. The human
genetic code, or genome, is 99.9 percent throughout the world. But while the   (27)  of our DNA is the same, what's left is responsible for our individual differences--in eye color or disease risk, for example.   (28)   very rare occasi tion, can occur, which is then passed down to all of that person's descen
sions, a small change, called a mutati ndants. Generations later, finding that same mutation in two people's DNA indicates that they share the same ancestor. By comparing mutations in many different populations, scientists can trace their ancestral connections.
       These ancient mutations are easiest to track in two places: in DNA that is passed from mother to
child (called mitochondrial DNA, or miDNA for short), and in DNA that travels from father to son
(known as the Y chromosome, the part of DNA that   (29)   a child will be a boy). By comparing
the mtDNA and Y chromoso somes of people from variou ous populations, geneticists can get a rough idea of where and when those groups   (30)   in the great migrations around the planct.
(Adapted from 'Human Journey," by James Shreeve: National Geographic Magazine, March 2006)
【題組】26.
(A) diverse
(B)unequal
(C) identical
(D)distinct