Organ, in biology, a group of tissues in a living organism that have been adapted to perform a specific function. In higher animals, organs are grouped into organ systems; e.g., the esophagus, stomach, and liver are organs of the digestive system.
You don't have the options to choose from but I believe from past experience the answer to the question is element.
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<em>Hand Washing</em> is one of the most important practices when it comes to avoid spreading germs due it helps removing microorganisms and potential pathogens that could be left on after regular activities which would alter the results of lab experiments, and also could easily remain after such lab experiments which could lead to the propagation of bacteria or harmful chemicals.
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