Can you answer the Situation For 50 points Problem Situation: Jimmy is reading a book for school that is 175 pages long. So far, He has read 50 pages. She can read 25 pages per hour. How many hours will it take her to finish the remainder of the book?

Which answer choice best represents the unknown quantity in this problem
A: h, the number of hours it will take to finish the book
B: p, the number of pages she can read per hour
C: b , the total number of pages in the book
D: a, the number of pages she has already read