At the surface of a particular ocean trench, the temperature is 18 degrees Celsius (${}^{\circ }$C). For every 100 meters of depth below the surface, the temperature decreases by approximately 0.5${}^{\circ }$C. Which of the following equations can be used to find the temperature $T$, in ${}^{\circ }$C, at a depth of $d$ meters below the surface?