Recent race:

Three boats on port tack pre start. A, B, C,

Lead leeward boat A, out of sight of trailing third boat C, tacks to starboard. Second boat, B, tacks to starboard. Third boat,C, readys to tack to lee bow of B. B falls off and A cracking of ahead of B comes into C's view. C's options: fall off hit B hard aft of mid ships bow on, or put helm hard over and be hit by A forward of C's mast while turning away from A. This was the option chosen by C. Nowhere else to go.

What rule apply and who is at fault?