The following SAS program is submitted:

%let a=cat;

%macro animal(a=frog);
    %let a=bird;
%mend;

%animal(a=pig)
%put a is &a;

Which one of the following is written to the SAS log?
(A) a is &a
(B) a is cat
(C) a is pig
(D) a is bird

Answer is B. But why D is not correct? Thanks.