Convection occurs in the troposphere but not in the stratosphere because
A) the troposphere is warmer than the stratosphere.
B) the troposphere is cooler than the stratosphere.
C) lower altitudes of the troposphere are warmer than higher altitudes, unlike in the stratosphere.
D) higher altitudes of the troposphere are warmer than lower altitudes, unlike in the stratosphere.
E) the troposphere contains fewer greenhouse gases than the stratosphere.
Question 2
When is/was gravitational contraction an important energy generation mechanism for the Sun?
A) It was important when the Sun was forming from a shrinking interstellar cloud of gas.
B) It is the primary energy generation mechanism in the Sun today.
C) It has played a role throughout the Sun's history, but it was most important right after nuclear fusion began in the Sun's core.
D) It is important during periods when the Sun is going from solar maximum to solar minimum.
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