Which of the following statement is true regarding Bohr's model of hydrogen atom?
(I) Orbiting speed of electrons decreases as it falls to discrete orbits away from the nucleus.
(II) Radii of allowed orbits of electrons are proportional to the principal quantum number.
(III) Frequency with which electrons orbit around the nucleus in discrete orbits is inversely proportional to the cube of principal quantum number.
(IV) Binding force with which the electron is bound to the nucleus increases as it shifts to outer orbits. 

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