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
Which of the following decreases in length during the contraction of a skeletal muscle fiber? 
Select and explain. 
A. A band of the sarcomere, B. I band of the sarcomere, C. Thick filaments, D. Thin filaments
B: I band of the sarcomere
Answer: B: I band of the sarcomere 
Explanation: The physical lengths of the actin and myosin filaments do not change during contraction. Therefore, the A band, which is composed of myosin filaments, does not change either. The distance between Z disks decreases, but the Z disks themselves do not change. Only the I band decreases in length as the muscle contracts.