Return one column with a groupby query - List<int> anonymous type error

Hi, I want to store the values of one column (ColumnD) as a List<int> to use it as a look up later on.

           List<int> query = (from t in db.TABLE where t.RecordID == recordID 
                            group t by new { t.ColumnA, t.ColumnB,t.ColumnC }
                            into grp
                            select new { grp.ColumnD}).ToList();

            return query;

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The code above gives me the following error:
'cannot implicitly convert from anonymous Generic.List type <..> to Generic List<int>'

if i return the query as an object instead, it works. Then I can look for a value using this:

var x = data.Find(a => a.ColumnD == [a value]);

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But how can I store the values of one column in a list??

Thanks!