If you leave a broken string unreplaced for even a day, you risk having the adjacent string break from increased strain. This is not well-known, but proven by experience. It is usually best to use the string one note lower if necessary. I always save usable strings after taking them off for emergency replacements. I recommend doing so. If your C’s and F’s break consistently, and sooner than the other strings, there is something wrong with your harp, probably a mechanical adjustment of some kind that is needed.