Which of the Following Correctly Describes the Reason for the Specificity

Which of the following correctly describes the reason for the specificity of valinomycin?

A) The K⁺ ion is the only ion small enough to fit through the ion channel.
B) The K⁺ radius is the ideal size for a large binding affinity because it minimizes steric strain and potential for degrees of rotation.
C) It selectively binds to the entrance of the transporter which is lined with negatively charged amino acids.
D) All of the above describe the specificity.
E) None of the above describe the specificity.