Where Is the Ion Selectivity Filter Located, Considering the Quaternary

Where is the ion selectivity filter located, considering the quaternary structure of a representative, well-studied ion channel like K_CS?

A) The outer vestibule of the channel, which is approximately 2 nm in diameter
B) Adjacent to short helices and their neighboring residues, which form a box-like structure at the channel's extracellular face
C) Within the central pore
D) Within the lower pore
E) By the inner helix, which predominantly forms the narrowest part of the channel structure