Explanation:
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The lungs belong to the respiratory.
Microtubules are composed of two different types of tubulin. Namely, these simpler units are called alpha-tubulin and beta-tubulin. These subunits are essentially tightly bound together to form heterodimers. In the microtubule structure, they are arranged in an organized manner in such a way that they in parallel direction with protofilaments.
Answer:
B,the variable that is changed and being tested
