An unfurnished classroom produces noticeable echoes when a teacher speaks. Which statement BEST explains this observation?
(a) An empty room decreases air resistance, so sound travels faster and returns as distinct echoes
(b) With no furniture to break up the wavefront, sound reflects back almost unchanged, making it seem louder and therefore echoing
(c) Soft furnishings absorb a portion of the sound energy; without them more sound is reflected by hard walls producing clear echoes
(d) Desks and curtains lower the frequency of the teacher’s voice; lower-frequency sounds do not produce audible echoes