An ionic atom equivalent to hydrogen atom has wavelength equal to 1/4 of the wavelengths of hydrogen lines. The ion will be
(a) ${\mathrm{He}}^{+}$         (b) ${\mathrm{Li}}^{++}$
(c) ${\mathrm{Ne}}^{9+}$        (d) ${\mathrm{Na}}^{10+}$

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