Question

_{(source: yfrog.com)}

I am trying my hand at making an invaders clone. there are 30 aliens arranged in a 5x 6 matrix on screen. I need to give the bottom most alien the ability to fire a shot.
I am using LINQ to group the aliens into 5 groups based on Location.X and then sort the groups descending.I then need to choose one of the groups ( that gives me 5 groups)
and select the First alien in the group and use it;s coordinate to fire a shot.

My code below ,well ,works , but aliens in ANY row are merrily firing shots- not just the bottom most. Pl look at my code below and tell me whats wrong.
(r = an instance of the Random class, all aliens are in a list called invaders).

 {
           var query = (from inv in invaders
                         group inv by inv.Location.X
                         into invgroup
                         orderby invgroup.Key descending 
                         select invgroup).ToList();

            var invfirst = query[r.Next(query.Count)].First();


                invaderShots.Add(new Shot(
                        new Point(invfirst.Area.X + invfirst.Area.Width / 2, invfirst.Area.Y + invfirst.Area.Height + 5),
                        displayrect, Shot.Direction.Down));

        }

EDIT:

Solved.Now it works as required after David B pushed me in the right direction.

Final code below.
May definitely need some improvements in light of the large number of grouping/sorting going on. If anyone has something constructive to say on this I am all ears ( or eyes , in this case). Thanks to all who helped.

   List<Invader> firstinvader = invaders.GroupBy(inv => inv.Location.X)
                .Select(g => g.OrderByDescending(inv => inv.Location.Y)).ElementAt(r.Next(5)).ToList();

            firstinvader.Sort(comparerByLocation);
            Invader item = firstinvader[firstinvader.Count -1];

            if(invaderShots.Count < 2)
            {
                invaderShots.Add(new Shot(
                     new Point(item.Area.X + item.Area.Width / 2, item.Area.Y + item.Area.Height / 2),
                     displayrect, Shot.Direction.Down));

            }
            else
            {
                return;

            }

Answer 1

List<Invader> firstInvaders = invaders
  .GroupBy(inv => inv.Location.X)
  .Select(g => g
     .OrderByDescending(inv => inv.Location.Y)
     .First())
  .ToList();

Invader shooter = firstInvaders[r.Next(firstInvaders.Count)];