sas macro usage (1)

Problem:
For the following macro, which usage(s) are correct?
%macro add_xsh(a, b=10);
%global result;
%let result=%sysevalf(&a+&b);
&result
%mend add_xsh ;

1. %let c=%add_xsh(3, b=20);
2. %let c=%add_xsh(3, 20);
3. %let c=%add_xsh(3);