The correct answer is (b). All of the Parties have followed the IPCC Good Practice Guidance decision tree for method selection for CO2 emissions from fuel combustion, and have either satisfied or exceeded the minimum requirements. Accordingly, all six of these Parties applied the reference approach and at least a Tier 1 Sectoral Approach in each applicable category. In all cases this was even done for “non-key” categories. Some Parties actually applied a Tier 2 or higher approach in all applicable categories.
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