Which of the Following Statements Regarding Membrane Transporters Is True

Which of the following statements regarding membrane transporters is true?

A) a porin is always open but only transports solutes in one direction
B) an ion channel selective for K⁺ will also allow Na⁺ through since both are small enough to fit through the channel
C) amphotericin is able to assemble inside a membrane in such a manner to form a channel through which K⁺ and Na⁺ flow, thus disrupting the ion gradient
D) K⁺ channels can work in both directions, allowing the neuronal action potential to move in multiple directions
E) none of the above