I agree completely with AF-C (or AF-A). Dynamic 9 point is fine for subjects that are moving on a very predictable path and moving at moderate speed. If you have lightning fast reflexes, I guess it would be fine for fast moving sporting events as well. If you're a reflexes are mortal, however, and/or your subject is going to be moving fast, or changing directions suddenly, I'd opt for D21.
Further, I'd use Aperture Priority, turn on Auto ISO and give it the full range from ISO 100 to ISO 6400 or set it to Auto and then crank the minimum shutter speed slider all the way to right ("Faster"). Keeping your shutter speed at twice the focal length is going to be critical if you want sharp focus so if you're shooting at 200mm your bare minimum shutter speed is going to need to be 1/400. For fast action sports, assuming you want really clean stop action, you're probably going to want to be shooting at shutter speeds around 1/500 or so. It kind of depends...
What sports exactly are you going to be shooting? I would approach a swim meet a little differently than I would a rugby match, if you know what I mean.
Also, don't feel like every shot has to have totally clean stop-action... Introduce a little motion blur to add a feeling of motion to the shot. A little motion blur can add a real dynamic to sports photography that absolute stop-action lacks. "Frozen" sports shots are fine, but jeez... Mix it up a little, experiment, push yourself to get something with some "WOW Factor".
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