Answer to Question 1
Sedimentary limestone, because a carbon atom may spend millions of years in the sedimentary/metamorphic rock reservoir, while it spends a short time (years to millennia) in the other reservoirs.
Answer to Question 2
Continents assemble into a supercontinent, being drawn toward cool regions of the asthenosphere. The supercontinent acts as an insulator, so the mantle heats up below the continent. The continent rises in response, and tension at the surface breaks its apart forming smaller continents. These smaller continents eventually disperse and then reassemble, being drawn again to cool asthenosphere.
