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Math·Algebra·Linear functions

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A family has money in an account for renting movies online. Each time the family rents a movie, the cost of the rental is withdrawn from the account, and each rental costs the same amount of money. The function $f(m)=21-3m$ gives the amount of money, in dollars, in the account after the family has rented $m$ movies. Which of the following represents the amount of money, in dollars, withdrawn from the account each time the family rents a movie?

A

$3m$

B

3

C

21

D

$21-3m$