This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Muscle, Subject name: Physiology
Auxotonic contraction of a muscle is: 
Select and explain. 
A. Same as isotonic contraction for different lengths of fibres, B. Based on the principle of "recruitment of motor units", C. Combination of isometric + isotonic contractions, D. Dynamic followed by adynamic contraction
B: Based on the principle of "recruitment of motor units"
Answer: B: Based on the principle of "recruitment of motor units" 
Explanation: Auxotonic contraction: is the one in which the strength of contraction goes increasing, as the contraction proceeds. This phenomenon is about a single contraction. As the contraction proceeds, more and more motor units may be recruited in the contraction, thus increasing the strength of contraction.This is in skeletal muscle. Staircase phenomenon: is phenomenon for multiple contractions;in which the strength of contraction goes increasing in successive contractions.