Two different DNA samples were isolated from biological specimens. Further experiments demonstrated that one of these (X) was composed of 30 % ~A , 30 % G , 20 % ~T and 20 % C but could not be cut by an exonuclease (an enzyme that cuts nucleotides from the ends of DNA). The second DNA sample (Y) could be cut by the exonuclease and was found to be composed of 30 % ~A , 30 % ~T , 20 % G and 20 % C . Which of the follow