Why do some stickleback populations lack pelvic spines? a) In seawater, pelvic spines help fish swim faster, but not in freshwater.
b) The pelvic spines are homologous to legs in four-legged animals. Because fish don't need hind limbs to walk, many populations of fish evolved to lack pelvic spines.
c) In lakes where there are no predatory fish, there is no advantage to having pelvic spines.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.

sound moves fastest in solids because the molecules are tightly packed and it allows sound to vibrate