The nervous system increases the force of muscle contraction in a graded fashion primarily by recruitment of active motor units and by increasing which of the following?
(A) Amplitude of action potentials of single motoneurons
(B) Duration of action potentials in single motor nerve fibers
(C) Frequency of firing of individual motoneurons
(D) Number of active end-plates on the muscle fiber
(E) Rate of synthesis of acetylcholine at the motor nerve terminal