Which of the following statements concerning potassium channel openers is true?

a) These agents decrease transmembrane calcium current associated in a smooth muscle with long-lasting relaxation and in a cardiac muscle with a reduction in contractility

b) These agents has a moderate reflex and vascular dilative action caused by the stimulation of sensitive nerve endings

c) Beneficial effects of these agents are related primarily to their hemodynamic effects – decreased heart rate, blood pressure, and contractility – which decrease myocardial oxygen requirements at rest and during exercise

d) These agents increase the permeability of K channels, probably ATP-dependent K channels, that results in stabilizing the membrane potential of excitable cells near the resting potential