下列程序段的执行结果为 a="ABBACDDCBA" For I＝6 To 2 Step-2 x＝Mid(a,I,I) y＝Leh(a,I) z＝Right(a,I) z＝x & y & z Next I Print z

A．ABA

B．AABAAB

C．BBABBA

D．ABBABA