How do I add a class to a given element

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<li class=”ListItem” data-pos=”0″>Correct Answer</li>
<li class=”ListItem” data-pos=”1″>Wrong Answer</li>
<li class=”ListItem” data-pos=”2″>Wrong Answer</li>
<li class=”ListItem” data-pos=”3″>Wrong Answer</li>
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Now I want to create a JavaScript function that will add a class to the existing class, whan click on it. (not replace, but add).

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<li class=”ListItem a0″ data-pos=”0″>Correct Answer</li>
<li class=”ListItem a1″ data-pos=”1″>Wrong Answer</li>
<li class=”ListItem a3″ data-pos=”2″>Wrong Answer</li>
<li class=”ListItem a4″ data-pos=”3″>Wrong Answer</li>
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Please Give The Answer How To, How can I do that?

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