When a teacher is talking in an enclosed air-conditioned classroom, the students do not hear echo because
(1) The wall is not big enough.
(2) Sound travels too fast.
(3) The sound energy is absorbed by the students and the furniture.
A(1) and (2) only
B(1) and (3) only
C(2) and (3) only
D(1), (2) and (3).
Proper terms should be used that make the meaning clearer