单项选择题

下列程序段的输出结果是
CLEAR
STORE 10 TO A
STORE 20 TO B
SET UDFPARMS TO REFERENCE
DO SWAP WITH A,(B)
A,B
PROCEDURE SWAP
PARAMETERS Xl,X2
TEMP=X1
Xl=X2
X2=TEMP
ENDPROC

A.10 20
B.20 20
C.20 10
D.10 10