A.   the Moon has been around longer than the Earth and has been exposed to more periods of meteor bombardment.

B.   the Moon experiences almost no erosion compared to the Earth, causing craters to remain much longer.

C.   the Moon is much more deficient in iron relative to the Earth, causing craters to form more easily.

D.   the Moon experiences frequent volcanic eruptions that cause craters.

E.   the Moon is more distant than the Earth, causing incoming rocks to hit it first.

A.   older than

B.   the same age as

C.   younger than

A.   By observing the Moon long enough from Earth, night after night, we are able to draw a map of its entire surface.

B.   At any given location on Earth, there are two high tides each day - one caused by the gravitational pull of the Sun, and the other by the gravitational pull of the Moon.

C.   High tide occurs on the side of the Earth nearest to the Moon, while low tide occurs on the opposite side of the Earth.

D.   The same half of the Moon's surface is perpetually dark (craters on that side never see sunlight), leading us to call it the "dark side of the Moon."

E.   The orbital period of the Moon around the Earth and the rotation period of the Moon around its axis are equal.

A.   Despite being the closest planet to the Sun, some parts of Mercury have a surface temperatures far below the freezing point of water.

B.   Earth's atmosphere plays a central role in keeping the planet from freezing cold temperatures.

C.   Venus is the hottest planet in our Solar System, mainly due to a greatly elevated greenhouse effect.

D.   Long ago, Mars probably had a thicker atmosphere than it does now, with sufficiently high temperatures and pressure to allow liquid water to exist on its surface.

E.   Earth's atmosphere allows the Sun's infrared radiation in but doesn't allow much visible light to escape, resulting in the greenhouse effect.

A.   Earth

B.   Venus

C.   Mars

D.   Mercury

A.   Mercury

B.   Mars

C.   Uranus

D.   Venus

A.   Helium

B.   Magnesium

C.   carbon monoxide

D.   carbon dioxide

A.   Neptune

B.   Uranus

C.   Saturn

D.   Mercury

A.   Saturn

B.   Mercury

C.   Venus

D.   Mars

A.   Jupiter

B.   Earth

C.   Venus

D.   Mercury