Primary succession a) is caused solely by human events. b) occurs on preexisting soil. c) occurs in areas without soil. d) becomes normalized by climate species.

C) Occurs in areas without soil.
Primary succession occurs in lifeless areas in which soil that is incapable of sustaining life is formed due to lava, sand dunes, and rocks left from a retreating glacier.

(1) The varying number of muscle fibers that contract
In most muscles, the number of muscle fibers in different motor units ranges from a few to hundreds. The nervous system can thus regulate the strength of contraction in a muscle by determining how many motor units are activated at a given instant and by selecting large or small motor unties to activate. 
some muscles are almost always partially contracted. In such muscles, the nervous system my alternate activation among the motor units, reducing the length of time any one set of fibers is contracted. Prolonged contraction can result in muscle fatigue due to the depletion of ATP and dissipation of ion gradients required for normal electrical signals.ing.
(2) varying the rate at which muscle fibers are stimulated 
single action potential produces a twitch lasting about 100 msec or less. If a second action potential arrives before the muscle fiber has completely relaxed, the two twitches add together, resulting in greater tension. Further summation occurs as the rate of stimulation increases. When the rate is so high that the muscle fiber cannot relax at all between stimuli, the twitches fuse into one smooth, sustained contraction called tetanus.