Question: Mark $(\sqrt{)}$ against the correct answer in the following:
The function $f(x)=x^{3}+6 x^{2}+15 x-12$ is
A. strictly decreasing on $R$
B. strictly increasing on R
C. increasing in $(-\infty, 2)$ and decreasing in $(2, \infty)$
D. none of these
Solution:
