Answer:
The correct option is b) By the time they get to be theories, they cannot be disproven. It's not necessary that the theories are always right. They can be proven wrong.
Explanation:
Yes. That's a true statement. Ultrasound is indeed the name given to sounds with frequencies above the human range of hearing.
Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans can't hear it.
<span>Distance is a scalar quantity that refers to "how much ground an object has covered" during its motion. Displacement is a vector quantity that refers to "how far out of place an object is"; it is the object's overall change in position.</span>
Answer:
<u>6.87 ft/s</u> is the rate at which the top of ladder slides down.
Explanation:
Given:
Length of the ladder is, 
Let the top of ladder be at height of 'h' and the bottom of the ladder be at a distance of 'b' from the wall.
Now, from triangle ABC,
AB² + BC² = AC²

Differentiating the above equation with respect to time, 't'. This gives,

In the above equation the term
is the rate at which top of ladder slides down and
is the rate at which bottom of ladder slides away.
Now, as per question, 
Plug in
in equation (1) and solve for
. This gives,

Now, plug in all the given values in equation (2) and solve for 

Therefore, the rate at which the top of ladder slide down is 6.87 ft/s. The negative sign implies that the height is reducing with time which is true because it is sliding down.