My function get QueryString from some Web page as a string. I need to parce it, to check, what strategy i must use.

Now my code looks ugly (i think so):

public QueryStringParser(string QueryString)
        {
            if (string.IsNullOrEmpty(QueryString))
            {
                this._mode = Mode.First;
            }
            else if (QueryString.Contains(_FristFieldName) && !QueryString.Contains(_SecondFieldName))
            {
                this._mode = Mode.Second;
            }
            else if (!QueryString.Contains(_FristFieldName) && QueryString.Contains(_SecondFieldName))
            {
                this._mode = Mode.Third;
            }
            else
            {
                throw new ArgumentException("QueryString has wrong format");
            }
        }

There must'n't be both FieldNames in one QueryString.

How to change this code to be mo readable.