下列程序段的执行结果为a="ABBACDDCBA"For I＝6 To 2 Step-2x＝Mid(a,I,I)y＝Leh(a,I)z＝Right(a,I) z＝x & y & zNext IPrint z
A．ABA
B．AABAAB
C．BBABBA
D．ABBABA