How to run sqldatasource1 by dropdownlist selectedindex ==0 and sqldatasource2 by dropdownlist selectedindex ==1

I have asp.net page . I have 2 stored procedure to search city and to search country. I use 2 SqlDataSource. the SqlDataSource1 for search city and the SqlDataSource2 for search country.my problem is that I want use dropdownlist1 for select type of search.when selected index is 0 the SqlDataSource1 be run. and when selected index is 1 the SqlDataSource2 be run.my code on click button event:

GridView1.DataSourceID = "SqlDataSource1"; GridView1.DataBind();