Problem with select tag in wordpress

Hi,
I wanted to add a drop down list to my wordpress post. So i put this in the text editor

<select>
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="mercedes">Mercedes</option>
  <option value="audi">Audi</option>
</select>

however once I switch to the visual editor and then back to the text editor, i see this,

<select><option value="volvo">Volvo</option></select>
<select><option value="saab">Saab</option></select>
<select><option value="mercedes">Mercedes</option></select>
<select><option value="audi">Audi</option></select>

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why is wordpress automatically appending select tags to each option value?