An object is in motion when
a.
you observe the object move.
b.
the object’s speed increases.
c.
the object’s displacement is greater than the distance traveled.
d.
the object changes position relative to a stationary reference point.

The correct answer is D. The object changes position relative to a stationary reference point.

Explanation:

Motion occurs when a specific body/object displaces or changes its position which implies the object moves from a reference point to a new one. This is usually explained as forces act over the object and make the object move or change from a stationary state to motion.

In physics, motion is widely studied according to scientific laws and this is related to main other concepts such as speed, velocity, direction, acceleration or inertia that as motion are commonly studied based on a reference system or point. According to this, it can be concluded an object is in motion if "the object changes position relative to a stationary reference point".

Higher frequency.

Explanation:

Sound are mechanical waves that are highly dependent on matter for their propagation and transmission.

Sound travels faster through solids than it does through either liquids or gases. A student could verify this statement by measuring the time required for sound to travel a set distance through a solid, a liquid, and a gas.

Mathematically, the speed of a sound is given by the formula:

Generally, the frequency of a sound wave determines the pitch of the sound that would be heard.

A shrill sound refers to a type of sound that is typically sharp, high pitched and as such has higher frequency.

Hence, shrill sound is of higher frequency.