A the sign shown in the picture is a no-fireworks sign and needs to be posted in the school library.

B, the sign shown in the figure is a fire-flammable material sign, which needs to be posted in the school library.

The signs shown in figure C are water-saving signs and need not be posted in the school library.

The sign shown in figure D is a fire extinguisher sign, which needs to be posted in the school library.

So choose C.