It means that you note down the observation at time t = t1 of some experiment which you are performing.
Answer:

Explanation:
given,
speed of wind, = 14 m/s
frequency of horn,f_s = 1000 Hz
speed of sound,V = 344 m/s
frequency heard by the listener
using Doppler effect
f_L is the frequency of the sound heard by the listener
f_s is the frequency of sound emitted by the listener
V is the speed of sound
v_L is the speed of listener
v_s is the speed of source
now,
considering the frame of reference in which wind is at rest now, both listener and the source will be moving at 14 m/s
now on solving we will get

hence, the frequency heard by the listener is equal to 1000 Hz
The answer to this question is d- circuit D
Two current processes that is used in ensuring that there is enough clean water is maintaining environmental sanitation in communities in which will help promote cleanliness and orderliness and another thing, having people to be educated with cleanliness and proper waste disposal as this will contribute to making the water clean and maintain it that way.