单项选择题

执行下面的程序，单击窗体后窗体消息框内显示结果是( )。 PRIVATE SUB FORM_CLICK() DIM STR AS STRING, K AS INTEGER STR="AB" FOR K = LEN(STR) TO 1 STEP -1 STR=STR&CHR(ASC(MID(STR, K, 1)) +K) NEXT K MSGBOX STR END SUB