有如下代码段：if (num >= 0) if (num == 0) System.out.println(“first string”);else System.out.println(“second string”);System.out.println(“third string”);若num为3，则输出结果为：A:third string B:second stringthird string C:first stringthird string D:first stringsecond stringthird string 答案: second stringthird string