${{W}_{d}}=\dfrac{1}{2}m.{{v}^{2}}=U.e$
$\to m.{{v}^{2}}=2.U.e$
$\to {{v}^{2}}=\dfrac{2.U.e}{m}$
$\to {{v}^{2}}=\dfrac{2\,\,.\,\,18,{{2.10}^{3}}\,\,.\,\,1,{{6.10}^{-19}}}{9,{{1.10}^{-31}}}$
$\to {{v}^{2}}=6,{{4.10}^{12}}$
$\to v={{8.10}^{7}}\,\,\,\left( m/s \right)$
$\to v={{8.10}^{4}}\,\,\,\left( km/s \right)$
$\to $ câu $B$