Only on the middle hill does the acceleration along the path decrease with time, for the hill becomes less steep as motion progresses. when the hill levels off, acceleration will be zero. on the left hill, acceleration is constant. on the right hill, acceleration increases as the hill becomes steeper. the speed increases in all cases as the ball rolls downhill.

answer: the correct answer for the fill in the blank is rods.

rods are the specialized photoreceptor cells that are present in the retina of human eye. they are responsible for vision in the night time ( also called scotopic vision). they have very low visual acuity and color distinguishing ability.

as they are responsible for vision in night time, therefore they are found predominantly in the nocturnal animals.