Which of the following statements is a tautology?

Which one of the following ecosystem types has the highest annual net primary productivity?

The coordination number of a metal crystallizing in a hexagonal close-packed structure is

The solution of the differential equation $x y^2 d y - (x^3 + y^3) d x = 0$ is

$A$ plane is parallel to two lines whose direction ratios are $(1, 0, -1)$ and $(-1, 1, 0)$ and it contains the point $(1, 1, 1)$. If it cuts the coordinate axes at $A$,$B$,and $C$,then the volume of the tetrahedron $OABC$ is (in cubic units):

If $f(x) = \left| \begin{array}{ccc} 2 \cos x & 1 & 0 \\ x - \frac{\pi}{2} & 2 \cos x & 1 \\ 0 & 1 & 2 \cos x \end{array} \right|$,then $f^{\prime}(\pi)$ is equal to