定义一个结构体数组，存放10个学生的学号，姓名，三门课的成绩

从键盘输入10个学生的以上内容，存入文件stud.txt，关闭文件

打开stud.txt文件，将数据读出，查看是否正确写入，关闭文件。

打开文件stud.txt文件，读出数据，将10个学生按照平均分数从高到低进行排序，

分别将结果输出到屏幕上和另一文件studsort.txt中。-Define an array of structures, storage of 10 students to learn numbers, names, classes are the results from the keyboard for more than 10 students in the content stored in file stud.txt, close the file open stud.txt file, the data read out to see if written correctly, close the file. Open the file stud.txt file, read data, the 10 students to sort according to the average scores from highest to lowest, respectively, the results output to the screen and the other documents studsort.txt in.