I assume the hole is threaded, for the same size threads as the bolts that go into your water bottle bosses. You can test this pretty easily. Anyway, lots of bikes come with this sort of setup, and a plastic cable-guide is supposed to be bolted onto the outside of the bottom bracket. Most bike shops have lots of these floating around and will give you one for free. If you can't get ahold of one, PM me and I can mail one to you. I've got three extras. Note that you'll need a short bolt, otherwise it'll run into the bottom bracket shell and stop being able to be tightened.
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"c" is not a unit that measures tire width