Why do sound waves echo more in some locations than in others?

Echo is phenomenon of hearing a sound more than once when that sound is reflected off some surface. So echo is an application of reflection of sound.
There is a condition for an echo to take place. The minimum distance between the source of sound and the reflecting surface needed for the echo must be greater than or equal to 17 m at room temperature ( speed of sound at room temperature is 340 m/s) . Some places are open and this minimum distance condition is met easily and thus the echoes are heard more often. Some laces are crowded, so this minimum distance is not met, so echo phenomenon doesn't take place.
The second condition for echo is that the time interval between the original sound and the reflected sound should be atleast 0.1 seconds.If this time is less than 0.1 sec,echos will not be heard.So at some place where this condition if time is not met,we cannot hear echos there.

unlike poles attract

the answer is b, have negligible mass