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The amount of gasoline stored at a fuel station is decreasing at a rate of $r(t)$ liters per hour (where $t$ is the time in hours).$\newline$What does $\int_{20}^{21} r(t) d t=-450$ mean?$\newline$Choose $1$ answer:$\newline$(A) During the first $20$ hours, the amount of gasoline at the fuel station decreased by $450$ liters.$\newline$(B) During the $21^{\text {st }}$ hour, the amount of gasoline at the fuel station decreased by $450$ liters.$\newline$(C) During the first $21$ hours, the amount of gasoline at the fuel station decreased by $450$ liters.$\newline$(D) During the $20^{\text {th }}$ hour, the amount of gasoline at the fuel station decreased by $450$ liters.