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?). 

Example Input: No antiviral therapeutics have yet received regulatory approval or demonstrated clinical efficacy.
Example Output: background

Example Input: For this purpose , we have been constructing a knowledge base by organizing various experimental and computational observations as a collection of if-then rules.
Example Output: purpose

Example Input: Recombinant HA proteins were expressed and purified as previously described ( Benjamin et al. , 2014 ) ELISA binding assays were performed using 384 well plates ( Maxisorp , Nunc ) with 0.5 µg/ml of purified HA protein as described in Table S2.
Example Output:
method