Concept: The unread pages decrease linearly as reading time increases.Explanation: The story book has 50 pages in total.The student reads 10 pages per hour, so in t hours, the number of read pages is 10t.Unread pages N = Total pages − Read pages.Therefore, N=50−10t.Rewriting this gives N=−10t+50.Check the relation: When t=5 hours, N=−10(5)+50=0, meaning the book is finished.When t=4 hours, N=−10(4)+50=10, meaning 10 pages remain unread.Thus, the correct equation is N=−10t+50.Answer:N=−10t+50 (Option B).