(2020 $\frac{2018}{2021}$ - 2019 $\frac{20182018}{20212021}$) : $\frac{2018}{2021}$ bằng: A. $\frac{1}{2018}$ B. 1 $\frac{3}{2021}$ C. $\frac{2018}{2021}$ D. $\frac{1}{2021}$

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