在从另一个下拉列表中选择值后填充下拉列表

I want to be able to populate a dropdown list(ddlExercise) after selecting a value from another dropdown list(ddlType) At the moment I am getting the values for the ddlType dropdown from a sql query which is populating the dropdown list but when I select something, the 2nd dropdown list remains empty. I have 3 values in the ddlType dropdown(Gym,Core,Cardio) and once I select one of these, I am using the dropdownlist.selectedvalue to retrieve its corresponding results. Here is my code:I want to be able to populate a dropdown list(d