Which Statement Is Correct for Twitch Summation

Multiple Choice

Which statement is correct for twitch summation?

A) It is a means by which weak muscle contraction may be accomplished.
B) It results from increasing the number of motor units that are firing within a muscle.
C) It results from increasing the frequency at which motor units are firing within a muscle.
D) A high level of calcium ion is not required in twitch summation.