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<html><head><script src="https://ajax.googleapis.com/ajax/libs/jquery/3.7.1/jquery.min.js"></script><script>$(document).ready(function(){ $("button").click(function(){ $("p").addClass(function(n){ return "par_" + n; }); });});</script><style>.par_0 { color: blue;}.par_1 { color: red;}</style></head><body><h1>This is a heading</h1><p>This is a paragraph.</p><p>This is another paragraph.</p><button>Add classes to p elements</button></body></html>