Which of the following best explains why the Sun's luminosity gradually rises over billions of years?
A) Fusion gradually decreases the number of independent particles in the core, allowing gravity to compress and heat the core, which in turn increases the fusion rate and the Sun's luminosity.
B) Nuclear reactions in the Sun become more efficient with time, so that each fusion reaction releases more energy when the Sun is old than when it is young; this in turn raises the Sun's luminosity.
C) The Sun's core gradually expands with time, and this expansion means there is more room for energy to be generated and hence increases the Sun's luminosity.
D) The planets need more and more energy to maintain any life on them as time goes on, and therefore the Sun must bet hotter.
A
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