Question89. If $f(x)$ is a differentiable function for all $x$ and has a relative minimum at $x=a$, which of the following must be true about $f(x)$ ? A. $f^{\prime}(a)=0$ B. $f^{\prime}(x)$ changes from positive to negative at $x=a$. C. $f^{\prime}(x)$ changes from decreasing to increasing at $x=a$. D. $f^{\prime}(x)$ changes from concave down to concave up at $x=a$.