#include<stdio.h>
void main(void)
{
    int a;
    printf("请输入学生成绩:");
    scanf("%d",&a);
        if (a>=90) {
            printf("该学生成绩等级为优\n",a);
        }
        else if(80<=a<90) {
            printf("该学生成绩等级为良好\n",a);
        }
        else if(70<=a<80) {
            printf("该学生成绩等级为中等\n",a);
        }
        else if(60<=a<70) {
            printf("该学生成绩等级为及格\n",a);
        }
        else if(a<60)  {
            printf("该学生成绩等级为不及格\n",a);
        }
}







为什么到80以下全部是良好

范围不能这样写，这是数学上的
80<=a&&a<90
这种格式

#include<stdio.h>
int main(void)
{
    int a;
    printf("请输入学生成绩:");
    scanf("%d",&a);
        if (a>=90)
         
            printf("该学生成绩等级为优\n",a);
         
           else
           {
             if((80<=a)&&(a<90))
                 printf("该学生成绩等级为良好\n",a);
                   else
                    {
                          if((70<=a)&&(a<80))
                          printf("该学生成绩等级为中等\n",a);
                         else
                         {
                         if((60<=a)&&(a<70))
                           printf("该学生成绩等级为及格\n",a);
                          else
                            {
                                if(a<60)
                              printf("该学生成绩等级为不及格\n",a);
                            }
                           
                         }
                        
                   }
                     
           }
            
              
              
}

#include<stdio.h>
void main()
{
    int a;
    printf("请输入学生成绩:");
    scanf("%d",&a);
        if (a>=90)
            printf("该学生成绩等级为优\n",a);
        else
           {
             if(80<=a)
                 printf("该学生成绩等级为良好\n",a);
             else
                {
                  if(70<=a)
                  printf("该学生成绩等级为中等\n",a);
                  else
                      {
                      if(60<=a)
                      printf("该学生成绩等级为及格\n",a);
                     else
                         {
                            printf("该学生成绩等级为不及格\n",a);
                          }   
                      }   
                }
                     
           }            
}

楼上的编译都没通过。

编译通过了啊

这样才能通过：
#include<stdio.h>
int main()
{
    int a;
    printf("请输入学生成绩:");
    scanf("%d",&a);
        if (a>=90)
            printf("该学生成绩等级为优\n",a);
        else
           {
             if(80<=a)
                 printf("该学生成绩等级为良好\n",a);
             else
                {
                  if(70<=a)
                  printf("该学生成绩等级为中等\n",a);
                  else
                      {
                      if(60<=a)
                      printf("该学生成绩等级为及格\n",a);
                     else
                         {
                            printf("该学生成绩等级为不及格\n",a);
                          }   
                      }   
                }
                     
           }            
}

你的是.cpp文件 我的是.c文件