Using design pattern to solve unordered selection of values from multiple DropDowns

I have a menu with multiple DropDowns. I added code but at present, it entirely resides in the code-behind file. I want to use any design pattern to handle in an easy and uncluttered way the variety of choices.

The report generation criteria is prepared as follows:

Report Type DropDown options include:

  1. Scheme Type-wise
  2. Scheme-wise
  3. District-wise
  4. Block-wise
  5. All

By default, only the first DropDown is enabled. Selecting an option from this DropDown, enables the respective DropDowns.

Not only this, values of Scheme, District and Block DropDowns also change when an item is selected from any of these DropDowns or Scheme Type DropDown using AJAX.

It involves a variety of SQL queries and Enabling/Disabling DropDowns often. My present code has become cluttered with many IF and EndIfs.

I want to know whether Observer pattern or any approach using Classes be used to simplify this operation. Any way to make this multiple selections and filling of DropDowns manageable and simple?

I want the solution in .NET-2.0 [C#].