Which of the following is NOT one of the ways that the body can increase...

Why does this seem like a trick question?
(1st post-lab question)
Which of the following is NOT one of the ways that the body can increase the force produced by a skeletal muscle?
A) a recruitment of more motor units and, thus, activation of more muscle fibers
B) application of high-frequency stimulation by a motor neuron
C) maintenance of elevated intracellular calcium levels, thus inducing wave summation
D) application of higher voltages to the whole muscle

I'm guessing B but D didn't increase force without repeated stimulus.
Help!