Ans: B
I must be true. For any non-zero numbers $x$ and $y$,
I must be true. For any non-zero numbers $x$ and $y$,
$\begin{array}{rcl}
x & < & y \\
-x & > & -y
\end{array}$
II may be false. Take $x=-2$ and $y=-1$.
Obviously, $x < y$. However, $\dfrac{1}{x^2} = \dfrac{1}{4}$ and $\dfrac{1}{y^2}=1$.
i.e. $\dfrac{1}{x^2}<\dfrac{1}{y^2}$.
III must be true. For any non-zero numbers $x$ and $y$,
$\begin{array}{rcl}
x & < & y \\
x^3 & < & y^3
\end{array}$
