The difference between Select and SelectMany

#region Terrible Object

var hasAllTheItems =
    new[]
    {
        new[]
        {
            new
            {
                Name = "Test"
            }
        },
        new[]
        {
            new
            {
                Name = "Test2"
            },
            new
            {
                Name = "Test3"
            }
        }
    };

#endregion Terrible Object

var a = hasAllTheItems.Select(x => x.Select(y => y.Name));
var b = hasAllTheItems.SelectMany(x => x.Select(y => y.Name));
var c = hasAllTheItems.Select(x => x.SelectMany(y => y.Name));
var d = hasAllTheItems.SelectMany(x => x.SelectMany(y => y.Name));

Assert.AreEqual(2, a.Count());
Assert.AreEqual(3, b.Count());
Assert.AreEqual(2, c.Count());
Assert.AreEqual(14, d.Count());