A uniformly charged ring of radius 3a and total charge q is placed in xy -plane centred at origin. A point charge q is moving towards the ring along the z-axis and has speed v at z = 4a. The minimum value of v such that it crosses the origin is:

• A. \(\sqrt {\frac{2}{{\rm{m}}}} {\left( {\frac{4}{{15}}\frac{{{{\rm{q}}^2}}}{{4\pi {\varepsilon _0}{\rm{a}}}}} \right)^{1/2}}\)
• B. \(\sqrt {\frac{2}{{\rm{m}}}} {\left( {\frac{1}{5}\frac{{{{\rm{q}}^2}}}{{4\pi {\varepsilon _0}{\rm{a}}}}} \right)^{1/2}}\)
• C. \(\sqrt {\frac{2}{{\rm{m}}}} {\left( {\frac{2}{{15}}\frac{{{{\rm{q}}^2}}}{{4\pi {\varepsilon _0}{\rm{a}}}}} \right)^{1/2}}\)
• D. \(\sqrt {\frac{2}{{\rm{m}}}} {\left( {\frac{1}{{15}}\frac{{{{\rm{q}}^2}}}{{4\pi {\varepsilon _0}{\rm{a}}}}} \right)^{1/2}}\)