This young tree could not have been damaged ______accident; I believe it was done _____ purpose.

A．by; on

B．for; with

C．by; with

D．for; on

以下程序运行后的输出结果是【 】。 main() {char c1,c2； for(c1=’’O’’,c2=’’9’’；c1＜c2；c1++,c2-) printf("%c%c",c1,c2)； printf("\n"); }