How does LINQ Single behave?

LinqSingleBehaviour.cs

class Program
{
    static void Main(string[] args)
    {
        var myListOfNone = new List<Dog>();
        var myListOfOne = new List<Dog>() { new Dog("Fido", 5) };
        var myListOfSeveral = new List<Dog>() { new Dog("Fido", 5), new Dog("Lassy", 14) };

        var b = myListOfNone.SingleOrDefault(); //Null

        var c = myListOfOne.Single(); //One

        var a = myListOfNone.Single(); //InvalidOperationException
        var d = myListOfSeveral.Single(); //InvalidOperationException
        var e = myListOfSeveral.SingleOrDefault(); //InvalidOperationException

    }
}