Aneuron’s repetitive firing rate is limited by an absolute refractory period, during which a new action potential cannot be generated. which event is chiefly responsible for this limit?
a. opening of k+ leak channels.
b. opening of voltage-gated na+ channels.
c. opening of transmitter-gated cation channels.
d. inactivation of voltage-gated k+ channels.
e. inactivation of voltage-gated na+ channels.

Inactivation of voltage-gated Na⁺ channels.

Explanation:

The refractory period occurs mainly due to the inactivation of voltage-dependent sodium (Na⁺) channels, which occurs during the peak of action potential and persists for most of the hyperpolarization period.  

These inactivated sodium channels cannot be opened even if the membrane potential crosses the threshold. Slow closure of voltage-dependent potassium channels, which results in hyperpolarization, also contributes to the refractory period by making membrane depolarization difficult (even after voltage-dependent Na⁺ channels have returned to their closed, activatable state).

the element that provides strength to the exoskeletons of clams and oysters is phosphorus. this is so because calcium carbonate is the main component in the shells.

explanation: