Answer: The virus weakens as time passes.
Explanation: The virus will weaken continuously during transmission, and will reduce its toxicity after several generations of transmission, without causing much harm to the human body.
Periosteum is the thin yet tough outermost layer of bones. Its many collagen fibers support the bone and firmly connect it to the surrounding structures. Osteogenic cells in the periosteum play a vital role in the growth and repair of bony tissue.
Answer:
c-ab is the only correct answer