The equilibrium position for a pendulum is straight down. If it moves through that position every second then its period is actually 2 seconds. This is because the period is how long it takes to go from one extreme and back again. It will pass through the equilibrium point twice when doing this. Once on the way down and again on the way back.
Density is mass over volume. Density is also a charecteristic property wich defines a substance and two different substance cant have the same charasteristic property.
Answer:
I believe its D visible light, infrared, ultraviolet
That depends on what the sound is traveling through, and the density
and temperature of the substance.
In dry air, at sea-level pressure and 68° F, the speed of sound is
about 340 meters per second (761 miles per hour).