You can think of dna as a great library of information that exists to do one thing only. what is that thing?
select one:
a. direct the building of different protein molecules
b. build other double helix models of information
c. tell the brain how to construct the body
d. store data about what worked and what didn't

The answer is A. DNA is the genetic code that eventually gets transcribed into RNA and that RNA is then translated into protein chains (polypeptides). Thus the ultimate purpose of DNA is to build these proteins.

B is wrong because while DNA itself exists in a double helix, its main function is not to build other double helix models. In fact, RNA exists in single helix form.

C is wrong because the brain does not construct the body. 

D is wrong because DNA is just a body of information, and not a historical record.