Which statement BEST describes how magnetic forces can be harnessed to increase the speeds at which trains go? Select one:

Magnetic forces can be used to create the electricity the train needs for a power source.

Magnetic forces increase the attraction between the tracks and the wheels so that the trains do not fly off the tracks at high speeds.

Magnetic forces create a positively charged field; the negatively charged train wheels are repulsed and this force drives the train forward.

Magnetic forces can raise the trains off the track, reducing frictional forces acting against the train; less work is needed to propel the train.

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?