a
\(\begin{array}{l}
2.5 \times {10^{ - 2}} \times 0.2 = \frac{1}{2} \times 20 \times {10^{ - 3}}\left\{ { - {V^2} + {1^2}} \right\}\\
5 \times {10^{ - 3}} = 10 \times {10^{ - 3}}\left( {1 - {V^2}} \right)\\
1 - {V^2} = \frac{1}{2}\,\,;\,\,{V^2} = \frac{1}{2}\,\,;\,\,V = \frac{1}{{\sqrt 2 }}\, = 0.7
\end{array}\)