Answer: “The resident time would be zero because carbon is released as soon as it is absorbed”
Explanation: Just took the quick check :) and also “balanced” gave me a key word that nothing is unequal so the release of carbon and the carbon being absorbed can’t be unequal (decrease or increase) so I makes sense for it to be zero, the moment carbon is released is the moment it becomes absorbed. Not a great scientific explanation but that’s what I thought it as in this tiny brain of mines!