William Astbury discovered that the X‑ray diffraction pattern of wool shows a repeating structural unit of approximately 5.2 Å along the length of the wool fiber. Choose the statement that explains the observed 5.2 Å length of the repeating structural unit in the wool polypeptide, α‑keratin.

a. The wool polypeptide α‑keratin exists primarily as a B‑form double helix, where the length of each complete turn of the helix is 5.2 Å.
b. The wool polypeptide α‑keratin consists primarily of α‑helices arranged as coiled coils, where the length of each complete turn of the helix is 5.2 Å.
c. The wool polypeptide α‑keratin has a fully extended backbone and does not contain any regular secondary structure elements.
d. The wool polypeptide α‑keratin consists of left‑handed helices arranged in a triple‑helix coiled coil. In left‑handed helices, the length each complete turn of the helix is 5.2 Å.