What is the cost，in cents，of using a certain fax machine to send n pages of a report if the total cost for sending the first k pages is r cents and the cost for sending each additional page is s cents(Assume that n＞k．)（）
A. r+s(n-k) B. r+s(n+k) C. rs(n+k) D. kr+s(n-k) E. kr+ns