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<!DOCTYPE html>
<html>
<head>
<style> 
option:checked {
  height: 100px;
  width: 100px;
}
</style>
</head>
<body>
<select>
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="opel">Opel</option>
  <option value="audi">Audi</option>
</select>
</body>
</html>