You are taking a photograph of a horse race. A shutter speed of 125 at /5.6 produces

Question:

You are taking a photograph of a horse race. A shutter speed of 125 at ƒ/5.6 produces a properly exposed image, but the running horses give a blurred image. Your camera has ƒ-stops of 2, 2.8, 4, 5.6, 8, 11, and 16.
(a) To use the shortest possible exposure time (i.e., highest shutter speed), which ƒ-stop should you use?
(b) What is the shortest exposure time you can use and still get a properly exposed image?