Five years ago, a reindeer died of anthrax infection in a frozen ground near the Arctic Circle, and a boy nearby died of anthrax recently. Some scientists speculate that global warming has made the Arctic Circle "permafrost. " Melting began, and Bacillus anthracis was making a comeback. At the same time, some "ancient viruses" such as smallpox and black death pathogens may be buried in this permafrost. Scientists are extremely worried about this. If the following statement is true, which one is the most helpful for scientists? ['Permafrost is the best place for bacteria to stay alive for a long time, which may last up to one million years.', 'Humans have coexisted with bacteria and viruses, such as Black Death and smallpox, and humans have evolved genes that fight them.', 'Global warming warms northern hemisphere countries, tropical diseases such as malaria begin to occur in the northern hemisphere.', 'If the pathogen has not been exposed to humans for a long time, our immune system will not be prepared for it.']