Sentence:  We assume that average total unit cost in the relevant region of operation is constant with respect to quantity produced ( the average cost curve is horizontal , and <WSD>therefore</WSD> is identical with the marginal cost curve ) , and is the same for every firm ( and therefore for the industry ) . 
Meaning of the word marked with <WSD> tags:  (used to introduce a logical conclusion) from that fact or reason or as a result