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@vincentt, as per our curriculum we only have 2 things mentioned
Liquidity characteristics – Low bid ask spread, High depth, Resilient market (values near to intrinsic values/ prices adjust quickly)
Factors contributing to Liquidity – Many buyer and sellers, diversity of opinion, information and investment needs, Readily accessible electronic/physical location and market integrity
In the question, by firms I think mean which are based out of selling and buying in markets (given the context)