= Solution
A <Givens rotation> is the identity outside rows and columns $p,q$, with the $2\times2$ block
$$
\begin{pmatrix}c&s\\-s&c\end{pmatrix},\qquad c^2+s^2=1.
$$
The block's transpose times itself is the identity, so the whole matrix satisfies $\boxed{\Omega^T\Omega=I}$ and is an <orthogonal matrix>. In particular it preserves the <Euclidean norm>. To annihilate the lower entry of a column pair $(a,b)^T$, take $c=a/r$, $s=b/r$, $r=\sqrt{a^2+b^2}$; the rotated pair is $(r,0)^T$. If both entries are zero, take the identity rotation.
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