Yes, lines 41-45 are the exact lines for answer choice (A). Well done! As you see in lines 41-45, "Proponents...argue that this particular strain is altered only by the removal of the gene responsible for the strain's propensity to cause frost damage, thereby rendering it safer than the phytopathogen from which it was derived."
Therefore, the proponents argue that ONLY the gene responsible for frost damage is altered, meaning ONLY ONE gene is different i.e. exactly what is said in answer choice (A).
Hope that was helpful. Let me know if you have any other questions!
