Mapping C# object to BsonDocument

I am relatively new to MongoDB. I have an object with the following definition

[BsonDiscriminator("user")]
public Class BrdUser
{
    [BsonId(IdGenerator = typeof(StringObjectIdGenerator))]
    public string ID { get; set; }

    [BsonElement("username")]
    public string UserNm { get; set; }

    [BsonElement("email")]
    public string EmailAdrs { get; set; }
    .
    .
    .
    public IList<Question> Questions{ get; set; } //<-- Un sure as to what Bson type should this be
}

Where Questions is another BsonDocument defined as :

[BsonDiscriminator("userques")]
public class Question
{
    [BsonId(IdGenerator = typeof(StringObjectIdGenerator))]
    public string ID { get; set; }

    [BsonElement("title")]
    public string Title{ get; set; }

    [BsonElement("desc")]
    public string Desciption{ get; set; }
}

My Question is while mapping, what attribute should I use so that the User object deserializes with the Question objects. There is no [BsonDocument] attibute in C# Driver.