窦房结4期自动去极化产生的主要原因是( )

A、K[img=~\/QImg\/2013-06-27-14-49-35\/image001.gif]外流逐渐增加
B、K[img=~\/QImg\/2013-06-27-14-49-35\/image001.gif]外流逐渐衰减
C、Na[img=~\/QImg\/2013-06-27-14-49-35\/image001.gif]内流逐渐衰减
D、Na[img=~\/QImg\/2013-06-27-14-49-35\/image001.gif]内流逐渐增加
E、Ca[img=~\/QImg\/2013-06-27-14-49-35\/image006.gif]内流逐渐衰减