TASK DEFINITION: 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?). 
PROBLEM: are deposited during embryogenesis in bone marrow and other organs as a backup population for adult tissue committed stem cells ( TCSCs ) and are involved in several processes related to tissue or organ rejuvenation , aging , and cancerogenesis.

SOLUTION: purpose

PROBLEM: surgical masks and homemade masks when worn during a variety of activities by healthy volunteers and a simulated patient.

SOLUTION: method

PROBLEM: The in-hospital mortality rate was lowest ( 33.5 % ) in the youngest patients ( age 18-29 years ) and highest ( 68.2 % ) in the oldest patients ( > 80 years , P < 0.001 ).

SOLUTION:
finding