Scientists approximate that there are 20 billion stars like our sun in the Milky Way. Why does the sun appear to be brighter than any other star in the Milky Way?
A. the sun is older than all of the other stars in the Milky Way
B. the sun is closer to the earth than any other star in the Milky Way
C. the sun is hotter than any other star in the Milky Way
D. the sun has more energy than any other star in the Milky Way

The earth's moon is unusually large. two popular theories of the moon's origin include the "sister world" hypothesis, which states that the moon formed from the same materials as the earth, near enough to the earth that they fell into orbit around each other. a second theory is the "capture" hypothesis, in which the moon formed elsewhere in the solar system, and the earth's gravity pulled it into its orbit. studies of what the moon is made of indicate that some of its materials had to come from the earth or from the same area of the solar system where the earth had formed. at the same time, the moon does not contain much of the material that makes up the earth's core, so the moon could not have formed from the same materials as the earth. how do the two facts above affect the described theories of the moon's origin? a. they show that scientists will never agree on where the moon came from. b. they show that more experiments on moon formation need to be done. c. they show that no theory accounts for the existence of the moon. d. they show that neither theory is complete and entirely correct.

Which part of earth’s surface receives the most direct rays from the sun? a) equator b) ocean c) poles d) mountains

The number to the right of an element's symbol (ex. c-12) identifies the of an isotope.