Phase 1 of Myocardium AP is mainly due to:
Phase 0 (depolarization), fast sodium channels open.
Phase 1 (initial repolarization), fast sodium channels close. 
Phase 2 (plateau), calcium channels open and fast potassium channels close.
Phase 3 (rapid repolarization), calcium channels close and slow potassium channels open.
Phase 4 (resting membrane potential) averages about −90 millivolts.
Key Concept:

Phase 1 of myocardium AP is due to K+ opening >> inactivation of Na+ channel.  
Phase 2 of myocardium AP starts with Ca++ entry.
Long duration of myocardium AP is due to phase 2.

​
K+ efflux
Na+ channel blockage
Ca2+ efflux
None of the above