Ans: A
I is correct. $f(x)>k$ means the part of $y=f(x)$ above the line $L$. And the $x$-coordinates of $A$ and $B$ are $1$ and $7$ respectively. Therefore, the solution is $x<1$ or $x>7$.
I is correct. $f(x)>k$ means the part of $y=f(x)$ above the line $L$. And the $x$-coordinates of $A$ and $B$ are $1$ and $7$ respectively. Therefore, the solution is $x<1$ or $x>7$.
II is correct. The roots of the equation $f(x)=k$ are the $x$-coordinates of the intersection points $A$ and $B$. i.e. $1$ and $7$.
III is incorrect. The mid-point of $AB$ is $(4,k)$. Therefore, the equation of the axis of symmetry of $y=f(x)$ is $x=4$.
