Solution: ① The picture shows a no-smoking sign, which can prevent the temperature from reaching the ignition point of the book and causing a fire. ② The picture shows the sign of fire-flammable substances, which has a warning function. ③ The picture shows the water-saving sign, which has nothing to do with fire prevention. (4) the picture is a sign of fire equipment, which has a warning effect on fire prevention. ⑤ is a traffic sign, so ⑤ is an icon that doesn't need to appear in the library.

So choose C.