In this task, you are given a sentence from the research paper and your task is to classify the given sentence into the following categories: Background (Why is this problem important? What relevant works have been created before? What is still missing in the previous works? What are the high-level research questions? How might this help other research or researchers?), Purpose (What specific things do the researchers want to do? What specific knowledge do the researchers want to gain? What specific hypothesis do the researchers want to test?), Method (How did the researchers do the work or find what they sought? What are the procedures and steps of the research?), or Finding (What did the researchers find out? Did the proposed methods work? Did the thing behave as the researchers expected?). 
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Question: a positive reaction could be obtained when an infected gill tissue sample that contained 430 viral copies/μg was diluted up to five orders of magnitude.

Answer: finding


Question: populations encountered distinct pathogens , and natural selection increased the prevalence of alleles that are advantageous in the new ecosystems in both host and pathogens.

Answer: background


Question: The independence of the SARS-CoV E enhancement effect on VLP production from its viral packaging capacity suggests a distinct SARS-CoV E role in virus assembly.

Answer:
finding