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Mary and Percy are both short story writers. Last year, they wrote a total of $21$ short stories, and Percy wrote $3$ fewer stories than Mary did. Which of the following systems of equations could be used to determine $m$, the number of short stories Mary wrote last year, and $p$, the number of short stories Percy wrote last year?$\newline$Choose $1$ answer:$\newline$(A) $m-p=21$$\newline$$$m+p=3$$$\newline$(B) $m-p=21$$\newline$$$m=3 p$$$\newline$(c) $m+p=21$$\newline$$$m-p=3$$$\newline$(D) $m+p=21$$\newline$$$p-m=3$$