2. $f(x)$ is an odd function, and when $x>0$, $f^{\prime}(x)>0, f^{\prime \prime}(x)>0$, then when $x<0$, $f(x)$ is
A. A monotonically increasing convex function
B. A monotonically decreasing convex function
C. A monotonically increasing concave function
D. A monotonically decreasing concave function
2. Special function method. Take $f(x)=x^{3}$, it is easy to know that $f(x)$ satisfies all the conditions given in the problem. When $x<0, f^{\prime \prime}(x)=$ $6 x<0$, so at this time $f(x)$ is a monotonically decreasing concave function, choose D.