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Which would roll down a slanted desktop more quickly, a ballpoint pen full of ink or a ballpoint pen after it has run out of ink? (Note: Assume that the total mass of an object does not affect the rate at which it rolls.)
A. The pen full of ink will always win the race.
B. The empty pen will always win the race.
C. The empty pen will win if the incline is steep, and the full pen will win if the incline is gentle.
D. The full pen will win if the incline is steep, and the empty pen will win if the incline is gentle.