Over time, what is the net effect of the star-gas-star cycle in the Milky Way?
A) The gas of the interstellar medium is continually depleted in elements heavier than helium.
B) The temperature of the interstellar medium varies between the cool 10-50 degrees (Kelvin) required to form stars and the 2000-30,000 degree (Kelvin) temperatures of the outer layers of the stars that form. But over time, more and more of the interstellar medium remains cool.
C) The total mass in the galaxy's interstellar medium is gradually increased, and the new gas is continually enriched in heavy elements.
D) The total mass in the galaxy's interstellar medium is gradually reduced, and the remaining gas is continually enriched in heavy elements.
E) There is no net effect to the cycle. Mass in the interstellar medium lost to star formation is exactly replaced by stellar winds and supernova explosions.
Question 2
Most lunar craters are volcanic in origin.
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