Studies assessing the amounts of different carbon isotopes (¹²C, ¹³C, and ¹⁴C) in the atmosphere were important because they provided evidence that human activities were in fact adding CO₂ to the atmosphere. This is an example of what type of relationship?

Question

Studies assessing the amounts of different carbon isotopes (12C, 13C, and 14C) in the atmosphere were important because they provided evidence that human activities were in fact adding CO2 to the atmosphere. This is an example of what type of relationship?

Studies assessing the amounts of different carbon isotopes (¹²C, ¹³C, and ¹⁴C) in the atmosphere were important because they provided evidence that human activities were in fact adding CO₂ to the atmosphere. This is an example of what type of relationship?