SqlDataReader Access - 'using' Statement Vs 'CommandBehaviour.closeConnection'

Can someone explain this to me?
Whats the difference  in "using" statement of C#  Vs  "SqlDataReader rdr = cmd.ExecuteReader(CommandBehaviour.closeConnection)"????
i thought both closes all the resources both "connection" as well "underlying datareader"but i have come to believe that CommandBehaviour.closeConnection leaves the 'datareader' open and closes the connection ONLY.Here is why..----------
i have the c# code in a class where i am creating a connection,executing stored procedure and returning the datareader.I am making use of 'using' statement something like this here

public class DB
{
public SqlDataReader getReader()
{
 using (SqlConnection conn1 = new SqlConnection(param1))
            {
                SqlCommand cmd1 = new SqlCommand(sproc, conn1);
                cmd1.CommandType = CommandType.StoredProcedure;
                conn1Open();
        
                SqlDataReader rdr = cmd1.ExecuteReader();
                return rdrMIS;
            }}

-this datareader is used in the function in a main code something like this and assign it to Dropdown list(just outlining the code)

function abc(){

DB db1 = new db1()

ddl1.datasource = db1.getReader();
ddl1.DataTextField = "val1";
ddl1DataValueField = "val2";
ddl1.DataBind(); }

this code gives me the error "Invalid attempt to FieldCount when reader is closed"

- I think this is  because the 'using' statment above,is closing the connection as well as datareader,so its not available in the calling function

-however if i remove the 'using' statment and change the line
 SqlDataReader rdr = cmd.ExecuteReader(CommandBehaviour.closeConnection) instead ;
It works fine.Why?Is it because CommandBehaviour.closeConnection closes the connection but datareader is OPEN and still available to the function?If so,do i need to close the reader explicitly in my function?..i never did it and it works fine..

- i like to use 'using' statement ..it is  clean and makes sure resource disposal is properly taken care of.But i would still like to use datareader and not dataset.Also i  want to handle all the database related task such as opening,closing of connection,datareader in a class and not in my calling function.

Whats the best way to handle all this concern.

Thanks for your help.