When miner encountering a fork, one fork has less difficulty but longer in length, while the other fork is more difficult but short. Which fork should miners choose 
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I assume you talk about accidental hard fork when two miners find the same block simultaneously. When this happens, later block decides which chain becomes the longer one. In this case, miner should choose the longest chain because if they mine the short one, that means they are mining a fork, which is not supported, won't be supported and has no idea to mine.?