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以下代码可以判断一个字符串是否是,空白处的代码应该是哪个选项? Scanner sc = new Scanner(System.in); String r = sc.next(); for(int i=0; i< r.length()/2;i++){ if(r.charAt(i) != r.charAt( )){ System.out.print("false"); return; } } System.out.print("true"); A.r.length()-i-1 B.r.length()-i C.r.length()-1 D.r.length()