A school wants to improve the acoustics of its assembly hall, where words sound "mixed up" due to excessive reverberation. Which ONE of the following single modifications would MOST effectively solve the problem?
(a) Installing additional high-power loudspeakers around the hall
(b) Painting all interior surfaces with glossy paint to make them smooth
(c) Fixing thick carpets over the entire floor area
(d) Suspending large metallic panels from the ceiling to direct sound toward the audience