How dynamic add css class to navagion bar?

Question

 if I have this code in a user control file(ascx), I want the page current visited has class "active" 

<ul>   

   <li class="active"><a href="a.html">Home</a></li>

   <li><a href="b.html">About Us</a></li>

</ul>

 

for example: when I visited b.html, the html source should be:

 

<ul>

<li ><a href="a.html">Home</a></li>

<li class="active"><a href="b.html">About Us</a></li>

</ul>