从键盘输入一个班（全班最多不超过30人）学生某门课的成绩，当输入成绩为负值时，输入结束。　　(1).较好的用户输入输出提示信息　　(2).使用子函数来实现上述各个功能　　(3).最好不要使用全局变量-From the keyboard a class (the whole class a maximum of more than 30 people) students a course grades, when the input results are negative, the input end. (1). Good user input/output message(2). To use child function to realize the above each function(3). Had better not use global variables