How to make group by in LINQ?

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;



namespace LinqAssignment

{

    public partial class Form1 : Form

    {

        class Student

        {

            public string Name { get; set; }

            public int Level { get; set; }

            public double Marks { get; set; }

        }

        List<Student> list = new List<Student>();

        public Form1()

        {

            InitializeComponent();

            list.Add(new Student() { Name = "Ahmad", Level = 2, Marks = 90 });

            list.Add(new Student() { Name = "Ahmad", Level = 1, Marks = 80 });

            list.Add(new Student() { Name = "Khaled", Level = 1, Marks = 95 });

            list.Add(new Student() { Name = "Amjad", Level = 3, Marks = 40 });

        }



        private void button1_Click(object sender, EventArgs e)

        {

            var result = from d in list select d;

            dataGridView1.DataSource = result.ToList();

        }



        private void button2_Click(object sender, EventArgs e)

        {

            var result = from d in list group d by d.Name;

            dataGridView1.DataSource = result.ToList();

        }

}

}

var result=from d in list select d.Marks>=50;