On hearing thunder, a person opens his mouth so that to equalize the air pressure on the eardrum of both the ears. This helps balance the pressure inside and outside the ears, reducing discomfort caused by sudden changes in atmospheric pressure, such as those accompanying thunder or other loud noises.
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On hearing thunder, a person opens his mouth so that to equalize the air pressure on the eardrum of both the ears (Option [B]). Thunder is often accompanied by a sudden change in atmospheric pressure. By opening the mouth slightly, the person can equalize the pressure inside and outside the ear canal, reducing discomfort or pain caused by the pressure difference. This action helps prevent the eardrums from being pushed inward or outward abruptly, which can occur during rapid changes in air pressure.
Options [A] and [C] are less likely reasons because fear does not directly relate to opening the mouth, and opening the mouth does not significantly affect sound reception compared to the function of equalizing pressure. Option [D] is incorrect as opening the mouth is not primarily intended to expel air. Therefore, among the options provided, equalizing the air pressure on both eardrums is the most plausible reason for opening the mouth upon hearing thunder.