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?). 

Q: National guidelines recommend that healthcare workers ( HCW ) who use N95 masks require fit testing.

A: background
****
Q: and observational study included 55 patients with COVID-19 who were transferred to Shenyang Sixth People 's Hospital between January 20 and March 15 , 2020.

A: method
****
Q: Both proteins contain a single transmembrane-spanning segment but are integrated in the membrane with opposite orientations.

A:
background
****