如何通过ASP.NET中的另一个下拉列表过滤下拉列表值，c＃

I have a simple question about filters. I have 4 dropdown lists which filter the data in my web application from MySQL database table. The first dropdown list is already selected showing only one project_id but the others drop down lists displays all the possible values from the database - this is what I made so far. I have a simple question about filters. I have