# An electron is moving in Bohr's fourth orbit. It's de-Broglie wave length is $\lambda$. What is the circumference of the fourth orbit ?

$\begin {array} {1 1} (a)\;\frac{2}{\lambda} & \quad (b)\;2\lambda \\ (c)\;4 \lambda & \quad (d)\;\frac{4}{\lambda} \end {array}$

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