To model the total cost \( t \) for a single order of \( c \) cartridges, we need to consider both the cost of the cartridges and the shipping fee.
The cost of the cartridges is given by the product of the number of cartridges \( c \) and the price per cartridge, which is \( 18.99 \). The shipping fee is a flat rate of \( 7.95 \).
Thus, the total cost \( t \) can be expressed as: \[ t = 18.99c + 7.95 \]
This matches option D.
So, the correct answer is: D. \( t = 18.99c + 7.95 \).
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