下列程序的运行结果是（）。 
Program main 
 implicit none
 integer ::a=1
   integer ::b=2
 call add ( a )
   Call add ( b)
 Write(*,*) a , b
 Stop
  end 
Subroutine  add (sum) 
Implicit none 
integer :: sum
   sum=sum+1 
Return 
end