Put the words into THREE groups

Put the words into THREE groups (/s/, /z/ and /iz/)
books, doors, windows, boards, clocks, bags, pencils, desks, schools, classrooms, students, teachers, couches, benches, glasses, brushes, maps, sinks, beds, caculators, villars, lamps.
Mình đang cần rất rất gấp, các bạn giúp mình.