Answer:
C. double helix
Explanation:
Watson and Crick discovered that DNA molecules are double-helical structures. According to them, each DNA molecule has two DNA strands that are wound around an imaginary axis to form a double helix. These two DNA strands are held together by complementary base pairing between their bases. They discovered that a purine base pairs with a pyrimidine base to maintain a constant distance between the two strands of a double helix.
