Read the following passage: "The concept of a 'carbon footprint' quantifies the total greenhouse gas (GHG) emissions caused directly and indirectly by an individual, organization, event, or product. It is typically expressed in equivalent tons of carbon dioxide (CO2e). Calculating a carbon footprint involves assessing emissions from various sources, such as transportation (e.g., driving, flying), energy consumption in homes and businesses (e.g., electricity, heating), and the production and disposal of goods and services. While CO2 is the most common GHG, other potent gases like methane (CH4) and nitrous oxide (N2O) are also included in the calculation, converted to their CO2 equivalent based on their global warming potential. Reducing one's carbon footprint often involves adopting more sustainable practices, like using renewable energy, improving energy efficiency, and opting for lower-emission transportation methods. The ultimate goal is to mitigate climate change by reducing the overall release of GHGs into the atmosphere." According to the passage, what is the primary unit of measurement used to express a carbon footprint?

1. AGallons of fossil fuels consumed
2. BKilograms of methane (CH4)
3. CTotal number of greenhouse gases released
4. DEquivalent tons of carbon dioxide (CO2e)