Sentence:  We assume that average total unit cost in the relevant region of operation <WSD>is</WSD> constant with respect to quantity produced ( the average 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:  have the quality of being; (copula, used with an adjective or a predicate noun)