What causes the apparent retrograde motion of the planets?
A) As Earth passes another planet, its gravitational pull slows down the other planet so that it appears to be traveling backward.
B) When planets are farther from the Sun, they move slower than when they are nearer the Sun; it is during this slower period that they appear to move backwards.
C) The other planets never really appear to move backward; the background stars shift due to Earth's revolution around the Sun.
D) As Earth passes another planet, the other planet appears to move backward with respect to the background stars, but the planet's motion does not actually change.
E) Apparent retrograde motion is an illusion created by turbulence in Earth's atmosphere.
Question 2
Which characteristic of Earth explains why we have an ultraviolet-absorbing stratosphere?
A) the existence of photosynthetic life
B) the existence of plate tectonics
C) the moderate surface temperature
D) the existence of oceans
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