If One Would Like to Use the Voltage Clamp Technique

If one would like to use the voltage clamp technique to record presynaptic calcium currents from a nerve terminal, why are tetrodotoxin (TTX) and tetraethylammonium (TEA) used?

A) TTX blocks potassium channels and TEA blocks sodium channels.
B) TTX blocks sodium channels and TEA blocks potassium channels.
C) TTX blocks calcium channels and TEA blocks potassium channels.
D) TTX blocks sodium channels and TEA blocks calcium channels.
E) TTX blocks sodium channels and TEA blocks chloride channels.