给定一个百分制的分数，输出相应的等级。

import java.util.*;
class Mark{
    public static void main(String []args){
        System.out.println("please input marks:");
        double mark;
        Scanner scanner = new Scanner(System.in);
        mark = scanner.nextDouble();
    if(mark<0||mark>100){
        System.out.println("输入有误！");
        System.exit(0);
        }
    if(mark>=90)
        System.out.println("this mark is grade \‘A\‘");
    else if(mark>=80)
        System.out.println("this mark is grade \‘B\‘");
    else if(mark>=70)
        System.out.println("this mark is grade \‘C\‘");
    else if(mark>=60)
        System.out.println("this mark is grade \‘D\‘");
    else
        System.out.println("this mark is grade \‘E\‘");
    }
}