A scientist is modeling the depletion of a reactant in a chemical solution. The concentration $C(t)$ over time $t$ is known to be a convex (concave up) function for all $t>0$. Identify which of the following qualitative descriptions of the concentration-time graph correctly represents $C(t)$.

Graph A: The concentration decreases over time, but the rate of decrease slows down, meaning the gradient $C^{\prime }(t)$ is strictly increasing towards zero.

Graph B: The concentration increases over time, but each subsequent increase is smaller than the last, meaning the gradient $C^{\prime }(t)$ is strictly decreasing.

Graph C: The concentration increases and then decreases, reaching a clear peak at a local maximum.

Graph D: The concentration decreases over time, and the rate of decrease accelerates, meaning the gradient $C^{\prime }(t)$ becomes more negative as $t$ increases.