The graph above shows the volume of water $V$, in liters, in a tank $t$ minutes after a faucet is opened. Which of the following equations correctly relates $V$ and $t$?

[Graph description: A line passes through the origin (0,0) and the point (3, 15). The x-axis is labeled 'Time (minutes)' and the y-axis is labeled 'Volume (liters)'.]