单项选择题

有如下程序:    * * *主程序:P.prg* * *       * * *子程序:P1.prg* * *    SET TALK OFF             X2=X2+1    STORE 2 TO X1,X2,X3         DO P2    X1=Xl+1                X1:Xl+1    DO PI                 RETURN    xl+x2+x3               * * *子程序:P2.prg* * *    RETURN               x3=x3+1    SET TALK ON              RETURN TO MASTER  执行命令DOP后,屏幕显示的结果为

A.3
B.4
C.9
D.10