The inner ear consists of two systems of canals which are namely cochlea and vestibular apparatus. The vestibular apparatus consists of three semicircular canals, cristae and two sacs called as saccule and utricle. Cochlea is the organ of hearing which contain mechanoreceptors for hearing. This makes option B incorrect. The three semicircular canals lie at right angle to each other in vestibular apparatus of inner ear and are filled with endolymph; this arrangement senses the change in movement (acceleration or deceleration) or position and maintain equilibrium. The mechanism includes movement of cupula in response to movement of endolymph within the canal causing the bending of cilia to generate the nerve impulse. This makes option A correct. Hammer is the one of the three ossicles of the middle ear which is concerned with amplification of air pressure while passing it from the inner surface of tympanic membrane via incus to stapes. This makes option C incorrect. The eustachian tube extends from each middle ear to nasopharynx and permits the equalization of air pressure. This makes option D incorrect. Correct answer is A.