When an Action Potential Travels Down the T Tubules of a Cardiac

When an action potential travels down the T tubules of a cardiac muscle cell, it _______, which in turn causes _______.

A) closes voltage-gated calcium channels and prevents calcium from leaving the cell; channels of the sarcoplasmic reticulum to open and allow calcium to enter the sarcoplasm
B) opens voltage-gated calcium channels and allows calcium to enter the cell; channels of the sarcoplasmic reticulum to close
C) causes changes in the conformation of the DHP receptors; ryanodine receptors to close
D) opens voltage-gated calcium channels that allow calcium to enter the sarcoplasm; channels of the sarcoplasmic reticulum to open and allow calcium to enter the sarcoplasm
E) causes changes in the conformation of the ryanodine receptors; opening of the DHP receptors