The systems of the Earth have both internal and external sources of energy. Which of the following are internal sources of Earth's energy? I. fusion reactions between hydrogen atoms in Earth's core
II. the decay of Earth's radioactive isotopes
III. the gravitational energy from Earth's original formation
IV. heat that is generated when water molecules spontaneously split in Earth's oceans
A.
I and IV only
B.
I, II, and III only
C.
II and IV only
D.
II and III only

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.