18.A new DNA strand only elongates in the 5' to 3' direction because
(A)Okazaki fragments prevent elongation in the 3(to 5' direction.
(B)DNA polymerase begins adding nucleotides at the 5' end.
(C)replication must progress toward the replication fork.
(D)the polarity of the DNA molecule prevents addition of nucleotides at the 3' end.
(E)DNA polymerase adds nucleotides only to the free 3' end.