How is matter converted into energy in the sun’s core? A. Helium isotopes decay, releasing energy in the process.
B. Hydrogen nuclei collide and fuse, releasing energy in the process.
C. Hydrogen nuclei collide and fuse, consuming energy.
D. Hydrogen nuclei split apart, releasing energy in the process.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".

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.