Which Statement Concerning Action Potentials in the Heart Is Correct

Which statement concerning action potentials in the heart is correct?

A) The rising phase of the action potential in cardiac autorhythmic cells is due to a rapid Ca²⁺ influx.
B) The rising phase of the action potential in cardiac contractile cells is due to a rapid K⁺ influx.
C) The plateau phase of the action potential in cardiac contractile cells is due to a rapidCa²⁺ influx.
D) The plateau phase of the action potential in cardiac contractile cells is due to a rapid K⁺ influx.