What important change in the Sun over the past four billion years is thought to be very important to understanding the climates of Venus, Earth, and Mars?
A) a gradual weakening of the solar wind with time
B) a gradual dimming with time
C) a gradual brightening with time
D) a gradual reduction in the amount of ultraviolet and X-ray radiation coming from the Sun
Question 2
Orion is visible on winter evenings in the northern hemisphere but not summer evenings because of
A) interference from the full Moon.
B) the tilt of Earth's axis.
C) the location of Earth in its orbit.
D) the precession of Earth's axis.
E) baseball on television.
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