Use These Figures to Answer the Corresponding Questions

Short Answer

Use these figures to answer the corresponding questions.
The structure labeled "4" is

a. part of a channel that allows Na+ ions to diffuse into the ECF
b. the activation gate for a gated K+ channel
c. the inactivation gate for a gated Na+ channel
d. part of a second messenger system
e. the activation gate for a gated K+ channel and part of a second messenger system