Which statement best describes the structure of a DNA molecule?

• A. A single-stranded helix composed of nucleotides, with bases exposed to the environment and no specific base-pairing rules
• B. Two antiparallel nucleotide strands twisted into a double helix, with sugar-phosphate backbones on the outside and complementary base pairs (A-T and G-C) on the inside
• C. A triple helix of nucleic acid strands in which each base pairs with two complementary bases
• D. Two parallel strands held together by covalent bonds between complementary bases, forming a straight ladder-like structure

Answer: (B)