The force produced by a single skeletal muscle fiber can be increased by which of the following

A、Increasingthefrequencyofstimulationofthefiber;
B、Increasingtheintensityofstimulationofthefiber;
C、Increasingthenumberofvoltage-gatedNa+channelsinthesarcolemma;
D、IncreasingthepermeabilityofthesarcolemmatoK+