Solution
= Solution
One choice is \b[the <C2v molecular point group>], $\{E,C_2(z),\sigma_{xz},\sigma_{yz}\}$. The additional operation is a second reflection, since
$$
C_2(z)\sigma_{xz}=\sigma_{yz}.
$$
A bent <water> molecule has this symmetry. Choose the $z$ axis along its bond-angle bisector and put the molecule in the $xz$ plane: reflection in $yz$ exchanges its two hydrogen atoms, while reflection in $xz$ fixes their positions.