Sodium and potassium ion channels have several negatively charged residues at the entry to the channel. On what basis do K+ channels specifically select for K+ ions? (In other words, why dont K+ channels enable Na+ ions to cross the membrane?)

A. ionic radius, Na+ is to small
B. energy cost, it is to energetically costly to dehydrate Na+
C. charge, Na is slightly more electronegative than K and does not interact with the channel carbonyl groups
D. charge, Na is slightly less electronegative than K and does not interact with the channel carbonyl groups
E. ionic radius, Na+ is too big