Why does this code changes location as I need it to

I have a weird question. I have been writing a code to change t开发者_开发技巧he view by a <select>'s onchange like this:

<% Html.BeginForm(); %>
<label for="id" >Automat:</label>
<%= Html.DropDownList("id", Model as SelectList, new { @onchange = "window.location.href = document.getElementById('id').options[document.getElementById('id').selectedIndex].value;" })%>
<% Html.EndForm(); %>

The selected value is numeric (i.e. 1,2,...).

Suddenly, I am able by changing the selected option to go from URL

http://localhost:58296/Content/ViewContent/2

to

http://localhost:58296/Content/ViewContent/3

.. And I really don't know why it works. Can anyone explain that to me please?

---

The selected index of the drop down list is a 0-based index of the items in the list.

<select>
  <option>Some Option 1</option> <!-- I have index 0 -->
  <option>Some Option 2</option> <!-- I have index 1 -->
  <option>Some Option 3</option> <!-- I have index 2 -->
</select>

You are literally telling the select list, "When you change, grab the selected index of the value, and change the very last part of the URL to that index."