In 2018, the California Air Resources Board announced that the state's total carbon emissions fell by 12 million tons in 2016, ______.

a. meeting the state's goal of reducing emissions back to 1980 levels four years behind its target date in 2014
b. meeting the state's goal of reducing emissions back to 1980 levels four years of its target date in 2020
c. meeting the state's goal of reducing emissions back to 1990 levels four years of its target date in 2014
d. meeting the state's goal of reducing emissions back to 1990 levels four years of its target date in 2020

d. meeting the state's goal of reducing emissions back to 1990 levels four years of its target date in 2020