on checkbox/radiobutto selected

How can I capture on Radiobutton/checkbox selection. I am trying to achieve like below.
aspx:
<tr> <td> a: </td>         <td> <asp:CheckBox ID="a" Text=Yes runat="server" OnCheckedChanged="CheckBoxSelected"/> </td>       </tr> 
<tr> <td> b :</td> <td>                 <asp:DropDownList ID="DropDownList1" runat="server">                                 <asp:ListItem>High</asp:ListItem>                  <asp:ListItem>Low</asp ...