The figure shows a sound wave as a function of time when the source is stationary with respect to the observer. An Illustration of a transverse wave and four crests and four troughs are labeled 1 s total.
Which shows the perceived sound wave heard by a stationary observer when the source moves toward the observer?

An illustration of a transverse wave and three crests and three troughs are labeled 1 s total.
An Illustration of a transverse wave that is taller than the previous. Three crests and three troughs are labeled 1 s total.
An Illustration of a transverse wave and four crests and four troughs are labeled 1 s total.
An Illustration of a transverse wave and seven crests and seven troughs are labeled 1 s total.

Conventional hot-water heaters consist of a tank of water maintained at a fixed temperature. the hot water is to be used when needed. the drawback is that energy is wasted because the tank loses heat when it is not in use, and you can run out of hot water if you use too much. some utility companies are encouraging the use of on-demand water heaters (also known as flash heaters), which consist of heating units to heat the water as you use it. no water tank is involved, so no heat is wasted. a typical household shower flow rate is 2.50 gal/min (9.46 l/min ) with the tap water being heated from 51.0 ˚f (10.6 ˚c) to 120 ˚f (48.9 ˚c) by the on-demand heater.what rate of heat input (either electrical of from gas) is required to operate such a unit, assuming that all the heat goes into the water?