Professor Rhodus Is Lecturing About Selective Permeability of Neurons

Multiple Choice

 Professor Rhodus is lecturing about selective permeability of neurons. What would he tell the class?

A) All ions can only travel in one direction across the membrane.
B) All molecules must cross the membrane via a designated channel.
C) Only certain molecules can cross the membrane freely.
D) Only sodium and potassium ions can cross the membrane.