SAT Question a31417d1: Nonlinear Functions answer and explanation with Desmos solution

Choice B is correct. A model for a quantity $C$ that decreases by a certain percentage per time period $t$ is an exponential equation in the form $C=I(1-\frac{r}{100}{)}^t$, where $I$ is the initial value at time $t=0$ for $r\%$ annual decline. It's given that $C$ is the number of millions of CDs sold in the United States and that $t$ is the number of years after $2005$. It's also given that $600$ million CDs were sold at time $t=0$, so $I=600$. This number declines by $15\%$ per year, so $r=15$. Substituting these values into the equation produces $C=600(1-\frac{15}{100}{)}^t$, or $C=600(0.85{)}^t$.