Question

a) a sound wave
reaches the ear,
and the reflected
wave reaches the
ear less than 0.1
seconds later.

b) the reflecting
source is
approximately
21.5 meters from
the person.

Explanation:

To determine the correct option, we analyze the concept of echo perception. The human ear can distinguish between the original sound and its echo only if the time interval between them is at least 0.1 seconds. If the reflected wave (echo) reaches the ear less than 0.1 seconds after the original sound, the ear cannot distinguish them, and this is related to the perception of sound and echo. Option A describes a situation where the time between the original sound and the reflected wave is less than 0.1 seconds, which is consistent with the condition for the ear not to perceive them as separate (like in the case of reverberation or when the distance is too small for a distinct echo). Option B's distance calculation: using the formula for distance of echo \( d=\frac{v\times t}{2} \) (where \( v = 340\ m/s \) speed of sound, \( t = 0.1\ s \) minimum time for echo perception), \( d=\frac{340\times0.1}{2}= 17\ m \). So a distance of 21.5 m would give a time \( t=\frac{2\times21.5}{340}\approx0.126\ s \), which is more than 0.1 s, so the echo would be distinguishable. So A is correct as it describes the condition where the time between sound and reflected wave is less than 0.1 s (no distinct echo perception).

Answer:

A) A sound wave reaches the ear, and the reflected wave reaches the ear less than 0.1 seconds later.