Sentence:  We assume that average total unit cost in the relevant region of operation is constant with respect to quantity produced ( the <WSD>average</WSD> cost curve is horizontal , and therefore 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:  total cost for all units bought (or produced) divided by the number of units