How to Insert a document with array of subDocuments into MongoDB by C# classes?

public class Entity
        {

            public string name { get; set; }
            public string family { get; set; }
            public List<sc> score { get; set; }
        }

        public class sc
        {
            public int Math{ get; set; }

            public int literature{ get; set; }
        }

var en = new Entity();
            var sc1 = new sc();
            var sc2 = new sc();
            sc1.Math= 14;
            sc1.literature= 15;
            sc2.Math= 16;
            sc2.literature= 19;
            en.score[0] = sc1;
            en.score[1] = sc2;
            en.name = "Esi";
            en.family = "Moja";
            collection.Insert(en);

Object reference not set to an instance of an object.